sim: testsuite: clean up arch specific toolchain settings

In a multitarget build, we process all targets in order, so make sure
the toolchain settings from one don't leak into the next.

sim/testsuite/example-synacor/allinsn.exp

@@ -17,6 +17,4 @@ if [istarget *] {
 	}
 	run_sim_test $src $all_machs
     }
-
-    unset LDFLAGS_FOR_TARGET
 }