Check `__SEH__` when checking if ARM EHABI should be implied,
similarly to 4a3722a2c3 / D126866.
Fix a warning by using the right format specifier (PRIxPTR instead
of PRIx64), and add a double->float cast in a codepath that hasn't
been built so far.
This is enough to make SEH unwinding of itanium ABI exceptions on
ARM mostly work - one specific issue is fixed in a separate follow-up
patch.
Differential Revision: https://reviews.llvm.org/D126867
7c0e93cb89