[clang][driver][NFC] Call IsARMBigEndain function only for isARM and isThumb.

IsARMBIgEndian function returns true only if:
  1. The triples are either arm or thumb and the
     commandline has the  option -mbig-endian
  2. The triples are either armeb or thumbeb.

Missing the checking of arm or thumb triples in the
first case pass through the --be8 endian flag to
linker For AArch64 as well which is not expected.
This is the regression happened from the previous
patch https://reviews.llvm.org/D154786.

It is better to refactor to only call IsARMBigEndian
for isARM and isthumb satisfying conditions which
keeps ARM and AArch64 separate.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D155808