[libomptarget][nfc] Move non-freestanding headers out of common

Summary:
[libomptarget][nfc] Move non-freestanding headers out of common

Lowers the bar for building deviceRTL.
Drops math.h entirely as it wasn't used and libm is a big dependency.

Reviewers: jdoerfert, ABataev, grokos

Reviewed By: jdoerfert

Subscribers: jvesely, openmp-commits

Tags: #openmp

Differential Revision: https://reviews.llvm.org/D77071