ARM: support high registers in __builtin_longjmp on WoA

Windows on ARM uses a pure thumb-2 environment.  This means that it can select a
high register when doing a __builtin_longjmp.  We would use a tLDRi which would
truncate the register to a low register.  Use a t2LDRi12 to get the full
register file access.  Tweak the code to just load into PC, as that is an
interworking branch on all supported cores anyways.

llvm-svn: 274815