[libc] Fix more inline definitions

Fix a bunch more instances of incorrect use of the `static`
keyword and missing use of LIBC_INLINE and LIBC_INLINE_VAR
macros. Note that even forward declarations and generic template
declarations must follow the prescribed patterns for libc code so
that they match every definition, all template specializations.

Reviewed By: Caslyn

Differential Revision: https://reviews.llvm.org/D154260