[libc++] Make __num_get_float hidden

It's an internal function and shouldn't be exported. It's also a source
of discrepancy in the published ABI list; these symbols aren't exported
for me on CentOS 7 or Ubuntu 16.04, leading to spurious check-cxx-abilist
failures.

Differential Revision: https://reviews.llvm.org/D27153

llvm-svn: 290503