[ARM] ARM-specific attributes should be accepted for big-endian

The ARM-specific C attributes (currently just interrupt) need to check
for both the big- and little-endian versions of the triples, so that
they are accepted for both big and little endian targets.

TargetWindows and TargetMicrosoftCXXABI also only use the little-endian
triples, but this is correct as windows is not supported on big-endian
ARM targets (and this is asserted in lib/Basic/Targets.cpp).

Differential Revision: https://reviews.llvm.org/D24245

llvm-svn: 281596