[ELF,PPC64] Scope checkPPC64TLSRelax to section and simplify TLS markers
checkPPC64TLSRelax detects TLS GD/LD without TLSGD/TLSLD markers
(generated from old IBM XL) and disables TLS optimization. Previously it
set a per-file flag (ppc64DisableTLSRelax). Now scope it in the section
being scanned.
In addition, simplify the R_PPC64_TLSGD/R_PPC64_TLSLD marker handling:
the redundant `sym.setFlags(NEEDS_TLSIE)` is unnecessary as the
preceding GOT_TLSGD relocation already sets it.