[libomptarget][amdgcn] Implement missing symbols in deviceRTL

[libomptarget][amdgcn] Implement missing symbols in deviceRTL

Malloc, wtime are stubs. Malloc needs a hostrpc implementation which is
a work in progress, wtime needs some experimentation to find out the
multiplier to get a time in seconds as documentation is scarce.

Reviewed By: ronlieb

Differential Revision: https://reviews.llvm.org/D89725