[LLVM] Add environment triple for 'llvm' (#117218)
Summary:
The LLVM C library is an in-development environment for running
executables on various systems. Similarly how we have `-gnu` to indicate
that we are using a GNU toolchain we should support `-llvm` to indicate
the LLVM C library. This patch only adds the basic support for the
triple and does not do any necessary clang changes to handle compiling
with it.
Fixes https://github.com/llvm/llvm-project/issues/117251
