[libc] Fix definition and use of LIBC_INLINE macro

LIBC_INLINE was doubly defined in two headers.  Define it only in
one place. Also update a few uses to make sure it's always placed
where a function attribute is valid and is used consistently on
every declaration of the same function in case the attributes used
in its definition must match on declarations and definitions.

Reviewed By: abrachet

Differential Revision: https://reviews.llvm.org/D150731