[AMDGPU] Switch data layout by triple environment amdgiz

Switch data layout by target triple environment amdgiz and amdgizcl indicating using of an address space mapping in which generic address space is 0.

amdgiz is for non-OpenCL environment where generic address space is 0.

amdgizcl is for OpenCL environment where generic address space is 0.

Differential Revision: https://reviews.llvm.org/D31211

llvm-svn: 298758