[HIP] Use default triple in llvm-mc for system ld

The Ubuntu system ld does not recognize the amdgcn-amd-amdhsa target.
Instead the host object with embedded device fat binary should not be
assembled by that triple. It should use default triple, so that the
object is compatible with system ld.

Reviewed By: yaxunl

Differential Revision: https://reviews.llvm.org/D83145