[HIP] Allow std::malloc in device function

D106463 caused a regression that prevents std::malloc to be
called in the device function, which is allowed with nvcc.

Basically the standard C++ header introducing malloc in
std namespace by using ::malloc. The device ::malloc
function needs to be declared before using ::malloc
to be introduced into std namespace.

Revert D106463 and add a test.

Reviewed by: Artem Belevich

Differential Revision: https://reviews.llvm.org/D150965