[libc][NFC] Replace static inline and inline annotations with LIBC_INLINE.

This is first of a few patches which will do similar mechanical changes.

LIBC_INLINE is a simple macro which is default defined as just `inline`.
The idea is that, different downstream contexts can define the macro as
suitable to their use case and context. For example, one can choose to
define LIBC_INLINE as `[[clang::internal_linkage]] inline`.

Reviewed By: lntue

Differential Revision: https://reviews.llvm.org/D142154