Modify target-check to also work on toolchain/ targets.
Switch to crt*-kernel.o variant instead of clobbering the original crt*.o.

Remove the old patch which caused crtstuff.c to always compile for mcmodel=kernel.
This was forcing crtbegin/end to a specific corner-case and they couldn't
be used for other things like linking modules etc.

The new patch creates a separate crt*-kernel.o version which has mcmodel=kernel set
and is installed along the normal crt*.o variants.

cc_link() gained a new :kernel option to select the matching crt*.o files.