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

[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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@281596 91177308-0d34-0410-b5e6-96231b3b80d8