Use 32/64_PCREL to replace a pair of ADD32/64 and SUB32/64.

Subtraction for labels that require static relocation
usually generates ADD32/64 and SUB32/64.

If subsy of BFD_RELOC_32/64 and PC in same segment,
and disable relax or PC at start of subsy or enable
relax but not in SEC_CODE, we generate 32/64_PCREL
to replace a pair of ADD32/64 and SUB32/64.