//build/toolchain/musl-host-gcc: implement

This is a cc_toolchain which runs on x86 systems with Linux/gcc and
targets Smalltown via static musl builds.

It is currently unused, but can be tested by trying to build any
cc_binary with
--crosstool_top=//build/toolchain/musl-host-gcc:musl_host_cc_suite .

Test Plan: This has been tested manually by running it against a simple cc_binary. Another revision on top of this will attempt to build mkfs.xfs with it.

X-Origin-Diff: phab/D623
GitOrigin-RevId: ebdf51ee76d9d5a7fd94725c66ef53783f787df7
diff --git a/build/toolchain/BUILD b/build/toolchain/BUILD
index 5bf53d2..78c4ae6 100644
--- a/build/toolchain/BUILD
+++ b/build/toolchain/BUILD
@@ -2,7 +2,17 @@
 
 # Toolchain definitions.
 #
-# We currently define a single custom toolchain: the host_cc toolchain suite.
+# We currently define two toolchains:
+#
+#  - //build/toolchain:host_cc_suite , which is a fully unhermetic host toolchain,
+#    that can be used to build tools for the host.
+#  - //build/toolchain/musl-host-gcc:musl_host_cc_suite , which combines the host's
+#    gcc compiler with a sysroot tarball that targets Smalltown. This can be used to
+#    build C libraries/tools for Smalltown.
+#
+
+# This file defines //build/toolchain:host_cc_suite.
+#
 # This is a C++ toolchain that uses GCC from the host at hardcoded paths. We
 # can get away with this, as currently the entire build is performed in a known
 # container (see: //scripts:create_container.sh). We define this toolchain so