[AMDGPU][GlobalISel][NFC] Use amdhsa target for flat/private tests (#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 #110219, which will generate different
code depending on the unaligned-access-mode.

Author: ritter-x2a