PECOFF: loosen another assumption of x86 only

Cache the machine type value of the linking context.  We need this in order to
calculate the virtual address of the atom when resolving function symbols.
Windows on ARM must check if the atom is a function and if so, set the Thumb bit
for the returned virtual address.  Failure to do so will result in an abnormal
exit due to a trap caused by invalid instruction decoding.  The same information
can be used to determine the relocation type that was previously being done via
is64 to select between x86 and x86_64.

llvm-svn: 218106