[ARM] Use dwarf exception handling on MinGW

Enabling and using dwarf exceptions seems like an easier path
to take, than to make the COFF/ARM backend output EHABI directives.
Previously, no EH model was enabled at all on this target.

There's no point in setting UseIntegratedAssembler to false since
GNU binutils doesn't support Windows on ARM, and since we don't
need to support external assembler, we don't need to use register
numbers in cfi directives.

Differential Revision: https://reviews.llvm.org/D39532

llvm-svn: 318510