sim/cris: Fix references to cgen cpu directory

sim/cris: Fix references to cgen cpu directory

Don't assume that cgen is located within the binutils-gdb tree.  We
already have CGEN_CPU_DIR and CPU_DIR defined, these are the cpu/
directory within cgen, and the cpu/ directory within binutils-cpu.

The cris target tries to find CPU_DIR relative to the cgen source
tree, which can be wrong when building with an out of tree cgen.

sim/cris/ChangeLog:

	* Makefile.in: Replace uses of CGEN_CPU_DIR with CPU_DIR, and
	remove the definition of CGEN_CPU_DIR.