[libc] Enable aliasing on AMDGPU targets

AMDGPU supports aliases now, so we can drop this case and leave it only
for the NVPTX target. Unfortunately it's unlikely that NVPTX will be
able to support this in the future due to their PTX language being very
limited.

Reviewed By: lntue

Differential Revision: https://reviews.llvm.org/D154704