Add tests for ARM Cortex-R8

Add command-line tests for ARM Cortex-R8 checking that the driver calls
clang -cc1 with the correct little-endian/big-endian, and ARM/Thumb triple.

Patch by Pablo Barrio <pablo.barrio@arm.com>

Differential Revision: http://reviews.llvm.org/D18052

llvm-svn: 263245