[openmp][nfc] Simplify macros guarding math complex headers

The `__CUDA__` macro is already defined for openmp/nvptx and is not used by
`__clang_cuda_complex_builtins.h`, so dropping that macro slightly simplifies
nvptx and avoids defining it on amdgcn (where it is likely to be harmful).

Also dropped a cplusplus test from a C++ header as compilation will have
failed on cmath earlier if it was included from C.

Reviewed By: jdoerfert, fodinabor

Differential Revision: https://reviews.llvm.org/D105221