[ELF] Add target-specific relocation scanning for PPC32 (#181517)
Implement PPC::scanSectionImpl, following the pattern established for
x86 (#178846) and PPC64 (#181496). This merges the getRelExpr and TLS
handling for SHF_ALLOC sections into the target-specific scanner,
enabling devirtualization and eliminating abstraction overhead.
- Inline relocation classification into scanSectionImpl with a switch
on relocation type, replacing the generic rs.scan() path.
- Use processR_PC/processR_PLT_PC for common PC-relative and PLT
relocations.
- Handle R_PPC_PLTREL24 inline with addend masking via processAux,
removing the EM_PPC special case from process().
- Handle TLS GD/LD/IE directly, eliminating handleTlsRelocation,
getTlsGdRelaxSkip, and adjustTlsExpr overrides. Use handleTlsIe
for TLS IE, and handleTlsGd for R_PPC_GOT_TLSGD16.
- Use R_DTPREL unconditionally for DTPREL relocations, removing
R_RELAX_TLS_LD_TO_LE_ABS (PPC32 was the only user).
- Move TLS relaxation dispatch from relocateAlloc into relocate,
removing the relocateAlloc override.
- Simplify getRelExpr to only handle relocations needed by
relocateNonAlloc and .eh_frame.