as: add option for generate R_LARCH_32/64_PCREL.

Some older kernels cannot handle the newly generated R_LARCH_32/64_PCREL,
so the assembler generates R_LARCH_ADD32/64+R_LARCH_SUB32/64 by default,
and use the assembler option mthin-add-sub to generate R_LARCH_32/64_PCREL
as much as possible.

The Option of mthin-add-sub does not affect the generation of R_LARCH_32_PCREL
relocation in .eh_frame.