[Hexagon] Handle FK_Data_8 fixups in ELF object writer (#192149)
Hexagon is a 32-bit little-endian target with no native 64-bit
relocation. Map FK_Data_8 fixups to R_HEX_32 (or R_HEX_32_PCREL) so that
8-byte data directives (.8byte, .quad) referencing external symbols
produce valid relocations instead of a fatal error.
Also improve the diagnostic for unrecognized fixup kinds by including
the fixup kind number in the error message.