cpu/arm/k60: Move -nostartfiles to LDFLAGS.

Fixes a harmless warning in Clang 3.5:
x86_64-pc-linux-gnu-clang-3.5.0: warning: argument unused during compilation: '-nostartfiles'

-nostartfiles only has an effect during linking.

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>
cpu/arm/k60: Implement synchronization primitives.

 - Implements a basic lock (untested).
 - Implements interrupt safe counter variables.

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>
cpu/arm/k60: LLWU use safe counters.

Make use of exclusive_increment and exclusive_decrement to protect the
counter variables used by the stop mode inhibitor macros.

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