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.

gcc/testsuite/lib/target-supports.exp

@@ -2880,6 +2880,12 @@ proc check_fork_available {} {
 	# tell as we're doing partial links for kernel modules.
 	return 0
      }    
+    if { [istarget cris-*-*] } {
+	# Compiling and linking works, and an executable running e.g.
+	# gcc.dg/torture/ftrapv-1.c works on now-historical hardware,
+	# but the GNU simulator emits an error for the fork syscall.
+	return [check_effective_target_hw]
+    }
     return [check_function_available "fork"]
 }