[llvm] [Clang] Add Picolibc environment type for triples (PR #169613)

[Keith Packard via llvm-commits]

keith-packard wrote:

Yeah, I think the --libc option is probably a simpler and easier to understand route here -- mashing stuff into the triple has you wading into all of that ambiguity while a new command line option is easy to document and works uniformly across all targets.

Note that you'll need to adjust the library search path, include path and default linker script selection logic when switching libraries.

Note that the choice of the C library also impacts the choice of the C++ library as any pre-compiled bits must match; does the C++ library selection logic also go through the multilib stuff?

https://github.com/llvm/llvm-project/pull/169613