cpu/arm/k60: Add C++ support.

This should be enough to support custom classes and objects, both
global and local.

Might need some changes to LDFLAGS if using new/delete in C++.

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>
C++: Add rules for building .cpp files.

Added default rules for building .cpp files using the C++ compiler.
Linking is still done using the C compiler, which is fine unless using
STL stuff (in that case you need to specify -lstdc++ or similar).

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>
cpu/arm/k60: Removed custom C and C++ compile rules.

No reason to have custom rules when they are not different from the
default rules in Makefile.include.

The original Makefile.include rules are even better because they handle
dependency generation better.

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