[libc] Standardize "standards" values in YAML files

Today we use "posix" and "POSIX" interchangeably. Likewise, "GNUextensions" and "gnu".

There are places in hdrgen (e.g. header description generation: https://github.com/llvm/llvm-project/blob/main/libc/utils/hdrgen/hdrgen/header.py#L45-L54) which assume a particular (lowercase) spelling, however.

We need to decide on the particular format, achieve uniformity across YAML files, and add hdrgen checks (or some kind of validation included in llvm-libc) to enforce correct spellings.