MC: check machine magic when applying offset adjustments

The values for the relocation type can (and do) overlap across various
architectures.  When performing an adjustment of the emitted relocation in the
final object file, check that the file magic matches the target for which the
relocation type is valid (e.g. a I386 relocation is only applied to an X86
object file, and an AMD64 relocation is only applied to an X86_64 object file).
This was noticed while adding support for ARM WinCOFF object file emission.

A test case for this is not really possible as the values for REL32 do not
overlap on I386 and AMD64, which is why this was never noticed in practice.  The
ARM WinCOFF emission is not yet ready to merge into the tree.