[AMDGPU][GlobalISel][NFC] Use amdhsa target for flat/private tests (llvm#110672)

As a proxy criterion, mesa targets have unaligned-access-mode (which
determines whether the hardware allows unaligned memory accesses) not
set whereas amdhsa targets do. This PR changes tests to use amdhsa
instead of mesa and inserts additional checks with unaligned-access-mode
unset explicitly.

This is in preparation for PR llvm#110219, which will generate different
code depending on the unaligned-access-mode.

(cherry picked from commit 16ba126,
resolved merge conflict in flat-scratch.ll)
Change-Id: I508d62e4f7b97615481e3955217fe8054e9a0dfc

Author: ritter-x2a