light up sanitizers for ARM

You can expect the sanitizers to be built under any of the following conditions:

1) CMAKE_C_COMPILER is GCC built to cross-compile to ARM
2) CMAKE_C_COMPILER is Clang built to cross-compile to ARM (ARM is default target)
3) CMAKE_C_COMPILER is Clang and CMAKE_C_FLAGS contains -target and --sysroot

Differential Revision: http://reviews.llvm.org/D3794

llvm-svn: 209835