sim/testsuite: PR29596, Trim extra path from arch

If subdir begins with "./", sim_arch returned "./ARCH" instead of
"ARCH" and target-specific check-sim is prevented.  This commit fixes
the issue (by trimming extra path from arch) to make "make check-sim" work.

sim/testsuite/lib/sim-defs.exp

@@ -14,7 +14,7 @@ proc sim_arch {} {
     while { [file dirname $arch] != "." } {
 	set arch [file dirname $arch]
     }
-    return "$arch"
+    return [file tail $arch]
 }
 
 # Initialize the testrun.