[libc] Fix compilation issues in memory_check_utils.h

Strict warnings require explicit static_cast to counteract
default widening of types narrower than int.

Functions in header files should have vague linkage (inline
keyword), not internal linkage (static) or external linkage
(no inline keyword) even for template functions.  Note these
don't use the LIBC_INLINE macro since this is only for test code.

Reviewed By: abrachet

Differential Revision: https://reviews.llvm.org/D151494