Make hip math headers easier to use from C

Summary:
Make hip math headers easier to use from C

Motivation is a step towards using the hip math headers to implement math.h
for openmp, which needs to work with C as well as C++. NFC for C++ code.

Reviewers: yaxunl, jdoerfert

Reviewed By: yaxunl

Subscribers: sstefan1, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D84476