[libc] Generate math.h instead of the static file it is currently.

Just enough to help a follow up patch adding cosf and sinf has been
added.

Reviewers: abrachet

Differential Revision: https://reviews.llvm.org/D76723

GitOrigin-RevId: 2c080a3f23e49edddb0799a569d7d3319fc91d86
5 files changed