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.
Core: switch error mapping to switches

Rather than a series of cascading ifs, use a switch statement to convert the
error code to a string.  This has the benefit of allowing the compiler to inform
us if we ever add a new error code but fail to update the string representation.

Add in stringified versions for a couple of missing InputGraphErrors.