Rename makefiles BUILD.mk

This way they appear at the top of directory listings.

libc/crt/BUILD.mk

@@ -0,0 +1,34 @@
+#-*-mode:makefile-gmake;indent-tabs-mode:t;tab-width:8;coding:utf-8-*-┐
+#───vi: set et ft=make ts=8 tw=8 fenc=utf-8 :vi───────────────────────┘
+#
+# SYNOPSIS
+#
+#   Cosmopolitan C Runtime.
+#
+# OVERVIEW
+#
+#   This is a special object library defining the Cosmopolitan SysV
+#   userspace runtime entrypoint, i.e. _start(). The $(CRT) argument
+#   should come first in the link order when building ELF binaries;
+#   becauseo it's the lowest level function; it wraps main(); and it
+#   decides which runtime support libraries get schlepped into builds.
+#
+#   Please note that, when building an αcτµαlly pδrταblε εxεcµταblε,
+#   lower-level entrypoints exist and as such, $(CRT) will generally
+#   come second in link-order after $(APE).
+
+PKGS += CRT
+
+CRT_ARTIFACTS += CRT
+CRT = o/$(MODE)/libc/crt/crt.o
+CRT_FILES = libc/crt/crt.S
+CRT_SRCS = libc/crt/crt.S
+CRT_OBJS = o/$(MODE)/libc/crt/crt.o
+$(CRT_OBJS): $(BUILD_FILES) libc/crt/BUILD.mk
+
+# these assembly files are safe to build on aarch64
+o/$(MODE)/libc/crt/crt.o: libc/crt/crt.S
+	@$(COMPILE) -AOBJECTIFY.S $(OBJECTIFY.S) $(OUTPUT_OPTION) -c $<
+
+.PHONY: o/$(MODE)/libc/crt
+o/$(MODE)/libc/crt: $(CRT)