[MC][X86] Make .reloc support arbitrary relocation types

Generalizes D62014 (R_386_NONE/R_X86_64_NONE).

Unlike ARM (D76746) and AArch64 (D76754), we cannot delete FK_NONE from
getFixupKindSize because FK_NONE is still used by R_386_TLS_DESC_CALL/R_X86_64_TLSDESC_CALL.