PECOFF: loosen assumptions about x86-only targets

Rather than saving whether we are targeting 64-bit x86 (x86_64), simply convert
the single use of that information to the actual relocation type.  This will
permit the selection of non-x86 relocation types (e.g. for WoA support).

Inline the access of the machine type field as it is relatively cheap (a couple
of pointer dereferences) rather than storing the relocation type as a member
variable.

llvm-svn: 218104