posix: separate option groups into c library ext and system interfaces

Separate the POSIX implementation into two categories:

- Extensions to ISO C
- System Interfaces

The first category include standalone functions that generally do not
require OS support or depend on any other features within the POSIX
specification. The Option Groups that comprise this category include

- POSIX_C_LIB_EXT: e.g. strnlen(), fnmatch()
- POSIX_C_LANG_SUPPORT_R: e.g. gmtime_r(), strtok_r()

The second category includes the majority of other POSIX Option Groups
that do require OS support. The latter group may also be categorized
generally as being NATIVE_LIBC_INCOMPATIBLE, although that might
eventually become more granular.

Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>