clang: Switch SunOS back to using the GCC runtime.

Upstream LLVM have removed the SunOS code from libcxx et al, despite us
offering to support it and it working fine, and so we're left with no
choice but to go back to using the GCC libraries and headers, which is a
shame.

Tested to compile at least a few different C/C++ files, and is able to
complete the builds of lang/wasi-*.

(jperkin)