GNAT/testsuite: Pass the `ada' option to target compilation

GNAT/testsuite: Pass the `ada' option to target compilation

Pass the `ada' option to DejaGNU's `target_compile' procedure, which by
default calls `default_target_compile', so that it arranges for an Ada
compilation rather the default of C.  We set the compiler to `gnatmake'
manually here, so that part of the logic in `default_target_compile' is
not used, but it affects other settings, such as the use of `adaflags'.

	gcc/testsuite/
	* lib/gnat.exp (gnat_target_compile): Pass the `ada' option to
	`target_compile'.

From-SVN: r276085