Make improvements

- You can now run `make -j8 toolchain` on Windows
- You can now run `make -j` on MacOS ARM64 and BSD OSes
- You can now use our Emacs dev environment on MacOS/Windows
- Fix bug where the x16 register was being corrupted by --ftrace
- The programs under build/bootstrap/ are updated as fat binaries
- The Makefile now explains how to download cosmocc-0.0.12 toolchain
- The build scripts under bin/ now support "cosmo" branded toolchains
- stat() now goes faster on Windows (shaves 100ms off `make` latency)
- Code cleanup and added review on the Windows signal checking code
- posix_spawnattr_setrlimit() now works around MacOS ARM64 bugs
- Landlock Make now favors posix_spawn() on non-Linux/OpenBSD
- posix_spawn() now has better --strace logging on Windows
- fstatat() can now avoid EACCES in more cases on Windows
- fchmod() can now change the readonly bit on Windows

bin/fatcosmoar

@@ -1,5 +1,4 @@
 #!/bin/sh
-
 PROG=${0##*/}
 MODE=${MODE:-$m}
 COSMO=${COSMO:-/opt/cosmo}
@@ -9,12 +8,18 @@ fatal_error() {
   exit 1
 }
 
-if [ ! -x "$COSMO/o/third_party/gcc/bin/x86_64-linux-musl-ar" ]; then
+if [ -x "$COSMO/o/third_party/gcc/bin/x86_64-linux-musl-gcc" ]; then
+  BRAND=musl
+else
+  BRAND=cosmo
+fi
+
+if [ ! -x "$COSMO/o/third_party/gcc/bin/x86_64-linux-$BRAND-ar" ]; then
   fatal_error "you need to run: fatcosmocc --update"
 fi
 
-AR_X86_64="$COSMO/o/third_party/gcc/bin/x86_64-linux-musl-ar"
-AR_AARCH64="$COSMO/o/third_party/gcc/bin/aarch64-linux-musl-ar"
+AR_X86_64="$COSMO/o/third_party/gcc/bin/x86_64-linux-$BRAND-ar"
+AR_AARCH64="$COSMO/o/third_party/gcc/bin/aarch64-linux-$BRAND-ar"
 if [ "$1" = "--version" ]; then
   # note: only the underlying gnu compiler binaries are gpl
   #       our shell script is released with the isc license