Add x86_64-linux-gnu emulator

I wanted a tiny scriptable meltdown proof way to run userspace programs
and visualize how program execution impacts memory. It helps to explain
how things like Actually Portable Executable works. It can show you how
the GCC generated code is going about manipulating matrices and more. I
didn't feel fully comfortable with Qemu and Bochs because I'm not smart
enough to understand them. I wanted something like gVisor but with much
stronger levels of assurances. I wanted a single binary that'll run, on
all major operating systems with an embedded GPL barrier ZIP filesystem
that is tiny enough to transpile to JavaScript and run in browsers too.

https://justine.storage.googleapis.com/emulator625.mp4

Makefile

@@ -60,7 +60,7 @@
 #
 #   build/config.mk
 
-SHELL = dash
+SHELL  = /bin/sh
 HOSTS ?= freebsd openbsd alpine
 
 .SUFFIXES:
@@ -69,8 +69,7 @@ HOSTS ?= freebsd openbsd alpine
 .PHONY: all o bins check test depend tags
 
 all:	o
-o:	o/libc					\
-	o/$(MODE)/ape				\
+o:	o/$(MODE)/ape				\
 	o/$(MODE)/dsp				\
 	o/$(MODE)/net				\
 	o/$(MODE)/libc				\
@@ -81,6 +80,7 @@ o:	o/libc					\
 
 PKGS =
 
+-include ~/.cosmo.mk				#──No.1
 include build/functions.mk			#─┐
 include build/definitions.mk			# ├──meta
 include build/config.mk				# │
@@ -89,6 +89,7 @@ include build/online.mk				# │
 include libc/stubs/stubs.mk			#─┘
 include libc/nexgen32e/nexgen32e.mk		#─┐
 include libc/intrin/intrin.mk			# │
+include libc/linux/linux.mk			# │
 include libc/math/math.mk			# ├──metal
 include libc/tinymath/tinymath.mk		# │
 include third_party/compiler_rt/compiler_rt.mk	# │
@@ -106,6 +107,7 @@ include libc/fmt/fmt.mk				# │
 include libc/rand/rand.mk			#─┘
 include libc/calls/calls.mk			#─┐
 include libc/runtime/runtime.mk			# ├──systems
+include libc/crt/crt.mk				# │
 include libc/unicode/unicode.mk			# │
 include third_party/dlmalloc/dlmalloc.mk	# │
 include libc/mem/mem.mk				# │
@@ -140,9 +142,9 @@ include third_party/editline/editline.mk
 include third_party/duktape/duktape.mk
 include third_party/regex/regex.mk
 include third_party/avir/avir.mk
+include third_party/ctags/ctags.mk
 include third_party/third_party.mk
 include libc/testlib/testlib.mk
-include libc/crt/crt.mk
 include tool/viz/lib/vizlib.mk
 include examples/examples.mk
 include third_party/lex/lex.mk
@@ -150,6 +152,8 @@ include third_party/m4/m4.mk
 include third_party/lz4cli/lz4cli.mk
 include third_party/bzip2/bzip2.mk
 include tool/build/lib/buildlib.mk
+include tool/build/emucrt/emucrt.mk
+include tool/build/emubin/emubin.mk
 include tool/build/build.mk
 include tool/debug/debug.mk
 include tool/decode/lib/decodelib.mk
@@ -213,7 +217,7 @@ depend:	o/$(MODE)/depend
 tags:	TAGS HTAGS
 
 o/$(MODE)/.x:
-	@$(MKDIR) $(dir $@) && touch $@
+	@$(MKDIR) $(@D) && touch $@
 
 o/$(MODE)/srcs.txt: o/$(MODE)/.x $(MAKEFILES) $(call uniq,$(foreach x,$(SRCS),$(dir $(x))))
 	$(file >$@) $(foreach x,$(SRCS),$(file >>$@,$(x)))
@@ -236,6 +240,71 @@ loc: o/$(MODE)/tool/build/summy.com
 	find -name \*.h -or -name \*.c -or -name \*.S | \
 	$(XARGS) wc -l | grep total | awk '{print $$1}' | $<
 
+COSMOPOLITAN_OBJECTS =		\
+	APE_LIB			\
+	LIBC			\
+	LIBC_ALG		\
+	LIBC_BITS		\
+	LIBC_CALLS		\
+	LIBC_CALLS_HEFTY	\
+	LIBC_CONV		\
+	LIBC_CRYPTO		\
+	LIBC_DNS		\
+	LIBC_FMT		\
+	LIBC_ELF		\
+	LIBC_LOG		\
+	LIBC_MEM		\
+	LIBC_NEXGEN32E		\
+	LIBC_NT			\
+	LIBC_OHMYPLUS		\
+	LIBC_RAND		\
+	LIBC_RUNTIME		\
+	LIBC_SOCK		\
+	LIBC_STDIO		\
+	LIBC_STR		\
+	LIBC_STUBS		\
+	LIBC_SYSV		\
+	LIBC_TIME		\
+	LIBC_TINYMATH		\
+	LIBC_UNICODE		\
+	LIBC_ZIPOS		\
+	THIRD_PARTY_DLMALLOC	\
+	THIRD_PARTY_DTOA	\
+	THIRD_PARTY_GETOPT	\
+	THIRD_PARTY_MUSL	\
+	THIRD_PARTY_REGEX
+
+COSMOPOLITAN_HEADERS =		\
+	LIBC			\
+	LIBC_CALLS		\
+	LIBC_CONV		\
+	LIBC_CRYPTO		\
+	LIBC_DNS		\
+	LIBC_FMT		\
+	LIBC_MEM		\
+	LIBC_RAND		\
+	LIBC_RUNTIME		\
+	LIBC_SOCK		\
+	LIBC_STDIO		\
+	LIBC_STR		\
+	LIBC_TIME		\
+	LIBC_UNICODE		\
+	LIBC_ZIPOS		\
+	LIBC_SYSV		\
+	LIBC_NT			\
+	THIRD_PARTY_DLMALLOC	\
+	THIRD_PARTY_DTOA	\
+	THIRD_PARTY_GETOPT	\
+	THIRD_PARTY_MUSL	\
+	THIRD_PARTY_REGEX
+
+o/$(MODE)/cosmopolitan.a: $(filter-out o/libc/stubs/exit11.o,$(foreach x,$(COSMOPOLITAN_OBJECTS),$($(x)_OBJS)))
+o/$(MODE)/.cosmopolitan.h: $(foreach x,$(COSMOPOLITAN_HEADERS),$($(x)_HDRS))
+	build/rollup $^ >$@
+o/$(MODE)/cosmopolitan.h: o/$(MODE)/.cosmopolitan.h
+	build/compile $(PREPROCESS) -P $(OUTPUT_OPTION) $<
+	clang-format-10 -i $@
+
 # UNSPECIFIED PREREQUISITES TUTORIAL
 #
 # A build rule must exist for all files that make needs to consider in
@@ -255,11 +324,12 @@ loc: o/$(MODE)/tool/build/summy.com
 # never get executed since they're not members of the transitive closure
 # of `make all`. In that case the build config could be improved.
 %.mk:
+~/.cosmo.mk:
 $(SRCS):
 $(HDRS):
 .DEFAULT:
 	@echo >&2
-	@echo NOTE: deleting o/$(MODE)/depend due to unspecified prerequisite: $@ >&2
+	@echo NOTE: deleting o/$(MODE)/depend because of an unspecified prerequisite: $@ >&2
 	@echo >&2
 	rm -f o/$(MODE)/depend