[MC] Refactor fixup evaluation and relocation generation
Follow-up to commits 5710759eb390c0d5274c2a4d43967282d7df1993
and 634f9a981571eae000c1adc311014c5c64486187
- Integrate `evaluateFixup` into `recordRelocation` and inline code
within `MCAssembler::layout`, removing `handleFixup`.
- Update `fixupNeedsRelaxation` to bypass `shouldForceRelocation` when
calling `evaluateFixup`, eliminating the `WasForced` workaround for
RISC-V linker relaxation (https://reviews.llvm.org/D46350 ).
