ld/PE: special-case relocation types only for COFF inputs

ld/PE: special-case relocation types only for COFF inputs

In 72cd2c709779 ("ld/PE: no base relocs for section (relative) ones") I
made a pre-existing problem quite a bit worse: When looking at a
relocation's (numerical) howto->type, that value is meaningful only if
the object was of corresponding COFF type. ELF objects in particular
have their own enumeration. As it stands, specifically the not entirely
unusual R_X86_64_32 and R_X86_64_32S did no longer have relocations
emitted for them, due to matching R_AMD64_SECTION and R_AMD64_SECREL in
value respectively.