Remove `clang: false` support

Most everything in Soong is using clang (and we're getting very close
for Make as well), so remove the ability to select GCC to build a
module.

We still need to keep the internal functionality around to find libgcc.a
and other toolchain_library modules with GCC.

Test: m blueprint_tools
Change-Id: I5ef3e3836b9ad3e160669ac32aee39698c9b72c3