testsuite: Tweak check_fork_available for CRIS

This takes care of the failing gcc.dg/torture/ftrapv-1.c and
-ftrapv-2.c for cris-elf.

For simplicity, assume simulators are the GNU simulator (in the gdb
repo).  But cris-elf is newlib, so a newlib target forking?  Yes: the
I/O, etc. interface to the simulator uses the Linux/CRIS ABI.

	* lib/target-supports.exp (check_fork_available): Don't signal
	true for CRIS running on a simulator.