gdb: Use UNSUPPORTED not XFAIL for unsupported target features

gdb: Use UNSUPPORTED not XFAIL for unsupported target features

If a target does not support making function calls from GDB then in a
number of test files, we currently report an XFAIL and skip some, or all
of the tests.  This commit changes the XFAIL to an UNSUPPORTED as this
seems more appropriate in these cases.

Some of the tests used bug ID 2416 to be reported in the XFAIL.  In the
current GDB bugzilla bug 2416 has nothing to do with calling target
functions from GDB.

gdb/testsuite/ChangeLog:

	* gdb.base/call-ar-st.exp: Report unsupported rather than xfail
	for unsupported target features.
	* gdb.base/call-rt-st.exp: Likewise.
	* gdb.base/call-sc.exp: Likewise.
	* gdb.base/call-signal-resume.exp: Likewise.
	* gdb.base/call-strs.exp: Likewise.
	* gdb.base/callexit.exp: Likewise.
	* gdb.base/callfuncs.exp: Likewise.
	* gdb.base/nodebug.exp: Likewise.
	* gdb.base/printcmds.exp: Likewise.
	* gdb.base/ptype.exp: Likewise.
	* gdb.base/structs.exp: Likewise.
	* gdb.base/unwindonsignal.exp: Likewise.
	* gdb.cp/gdb2495.exp: Likewise.
	* gdb.cp/templates.exp: Likewise.
	* gdb.cp/virtfunc.exp: Likewise.
	* gdb.threads/hand-call-in-threads.exp: Likewise.
	* gdb.threads/interrupted-hand-call.exp: Likewise.
	* gdb.threads/thread-unwindonsignal.exp: Likewise.