[libc] Add LIBC_INLINE_VAR for inline variables

These are the only variables I could find that use LIBC_INLINE. Note, these are namespace scoped constexpr so local linkage is implied. inline is useful here to silence clang's unused-const-variable variable. For Fuchsia, the distinction between LIBC_INLINE and LIBC_INLINE_VAR is helpful because we define LIBC_INLINE as `[[gnu::always_inline]] inline` when building with gcc. This isn't meaningful on variables.

Alternatively, we could make these variables simply constexpr and also add `[[maybe_unused]]`

Reviewed By: sivachandra, mcgrathr

Differential Revision: https://reviews.llvm.org/D152951