cpu/arm/k60: Add support for building with LLVM/Clang

Set BUILD_WITH_CLANG=1 in the build to build using Clang instead of
GCC.

either:
make BUILD_WITH_CLANG=1
or:
export BUILD_WITH_CLANG=1
make

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>