[MC][ARM] Make .reloc support arbitrary relocation types

Generalizes D61992. In GNU as, the .reloc directive supports arbitrary relocation types.

A MCFixupKind value `V` larger than or equal to FirstLiteralRelocationKind
is used to represent the relocation type whose number is V-FirstLiteralRelocationKind.

This is useful for linker tests. Without the feature the assembler
cannot produce certain relocation records (e.g.  R_ARM_ALU_PC_G0/R_ARM_LDR_PC_G0)
This helps move forward D75349 and D76575.

Differential Revision: https://reviews.llvm.org/D76746