[AMDGPU][True16][CodeGen] add fake16 to gisel test (#138588)
This is a NFC patch.
Add '-mattr=-real-true16' to gfx11/gfx12 test.
GISEL is not fully supported in true16 mode yet. However we might want
to turn on true16 mode for SDAG as default first. This patch is
preparing for this mode shift in the short future so we can have a small
patch to turn it on