testsuite: Verify that module-mapper is available

For some test cases, it's required that the optional module mapper
"g++-mapper-server" is built. As the server is not required, the
test cases will fail if it can't be found.

gcc/testsuite/ChangeLog:

	* lib/target-supports.exp (check_is_prog_name_available):
	New.
	* lib/target-supports-dg.exp
	(dg-require-prog-name-available): New.
	* g++.dg/modules/modules.exp: Verify avilability of module
	mapper.

Signed-off-by: Torbjörn SVENSSON <torbjorn.svensson@foss.st.com>