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

[via llvm-commits]

quic-k wrote:

@keith-packard @petrhosek 
thanks for review

the RFC was a while back, unsure if @voltur01 has any plans to work on it anytime soon

but if I had to implement it, I would introduce a clang flag say `--libc`, this flags value can be read and used by all the toolchain drivers in clang,
each driver can choose to ignore it (this is what all drivers will be doing initially)
if a toolchain driver decides to use it, they can leverage this flag to chose include/library paths, set crt0 names, etc.
I would use this flag for Hexagon toolchain driver

I don't see a way to handle this in a generic way for all tooclhain drivers

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