aarch64: Fix ldst_lo12_determine_real_reloc_type

aarch64: Fix ldst_lo12_determine_real_reloc_type

Add fallback handling for an invalid choice of opd0_qlf, instead of
hitting an assert when trying to use X registers in byte or half
instructions.

Additionally, simplify the code by inlining the relevant parts of
aarch64_get_expected_qualifer, and by deducing the array index directly
from the qualifier enum values (instead of looking up the element size
and computing its log).

This makes aarch64_get_expected_qualifier unused, so remove it.