[libc] Add definitions of double_t and float_t to math.h.

This change does not handle any extensions. Only the C standard
variations are handled.

Reviewers: abrachet

Differential Revision: https://reviews.llvm.org/D79150

GitOrigin-RevId: 245cbd15a49e57c6090254f5e114c80f796dc05c
3 files changed