ReaderWriter: print magic in hex

When we encounter an unknown machine type, we print out the machine type magic.
However, we would print out the magic in decimal rather than hex.  Perform this
conversion to make it easier to identify what machine is unsupported.
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.
Driver: accept /machine:arm for Windows linker

Accept /machine:arm as an argument.  This is changed to support ARM NT.
Although there is no way to differentiate between ARM (Windows CE) and ARM NT
(Windows on ARM), since LLVM currently only supports Windows on ARM, simply take
/machine:arm to mean Windows on ARM.
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.