gdb: Only run scheduler-locking tests if feature is supported

Not all targets support scheduler-locking.  Add a check to see if the
taraget supports scheduler locking, and if it doesn't, don't run the
scheduler-locking tests that will otherwise fail.

There are actually a set of tests that try to use scheduler-locking
however, in most of these cases the test will not be run on smaller
targets (those that might not support threads and scheduler-locking)
due to the targets lack of support for threads, or some other larger
feature.

In the gdb.mi/mi-cmd-param-changed.exp test though, there's no
dependence on threads, or any other larger feature, and so, for the
small target I was using the test would otherwise try to run, only to
fail due to lack of support for scheduler-locking.

gdb/testsuite/ChangeLog:

	* lib/gdb.exp (target_supports_scheduler_locking): New proc.
	* gdb.mi/mi-cmd-param-changed.exp: Only run scheduler locking
	tests if the target supports scheduler locking.