bfd/ELF: fold BFD_RELOC_<arch>_PCREL*

There's no need to have a separate reloc per arch; just like for other
more or less generic ones a single one will (mostly) do, as long as the
resulting reloc's properties fit such a generic use (in the assembler it
could, after all, also result from ordinary expressions or uses with the
.reloc directive). Arm64, C-Sky, and KVX - sadly - are once again
exceptions.

For cris it's a PLT reloc which is being replaced.

For msp430 also drop BFD_RELOC_MSP430_16_BYTE, which has already been
merely an alias of BFD_RELOC_16 (resolving to R_MSP430_16_BYTE).