lib/clang: Commit cleaned-up workaround for building on RISC-V

There is a long-standing issue on RISC-V around --gc-sections when
statically linking, which affects the bootstrap build of LLVM. Since
fixing this properly has remained a TODO for years, commit the
workaround with documentation so that building natively from source
works out of the box.

PR: 258358
MFC after: 1 week