Don't defer to the GCC driver for linking arm-baremetal

Also comes with a cmake cache for building the runtime bits:

 $ cmake <normal cmake flags> \
   -DBAREMETAL_ARMV6M_SYSROOT=/path/to/sysroot \
   -DBAREMETAL_ARMV7M_SYSROOT=/path/to/sysroot \
   -DBAREMETAL_ARMV7EM_SYSROOT=/path/to/sysroot \
   -C /path/to/clang/cmake/caches/BaremetalARM.cmake \
   /path/to/llvm

https://reviews.llvm.org/D33259

llvm-svn: 303873