PE/COFF: remove another use of PECOFFLinkingContext::is64bit

In order to support more than x86/x86_64, we need to change the behaviour to use
the actual machine type rather than checking the bitness and assuming that we
are on X86.  This replaces the use of is64bit in applyAllRelocations with a
check on the machine type.  This will enable adding support for handling ARM
relocations.

Rename the existing applyRelocation methods to be similarly named and to make it
clear the types of relocations they will process.

llvm-svn: 219088