The two-tier relaxation hook design (fixupNeedsRelaxationAdvanced
delegating to fixupNeedsRelaxation) is confusing and unnecessary.
Only M68k and AMDGPU used fixupNeedsRelaxation (via the default
fixupNeedsRelaxationAdvanced). AArch64 and CSKY had dead overrides:
they don't override mayNeedRelaxation to ever return true.
Merge the two hooks: convert M68k and AMDGPU to override
fixupNeedsRelaxationAdvanced directly, remove dead overrides from
AArch64 and CSKY, and simplify the default implementation to
`return !Resolved`.
56cdd60a2f