Add tests for C/C++ musttail attributes

Add tests for C/C++ musttail attributes

Some adopted from the existing C musttail plugin tests.
Also extends the ability to query the sibcall capabilities of the
target.

gcc/testsuite/ChangeLog:

	* lib/target-supports.exp:
	(check_effective_target_struct_tail_call): New function.
	* c-c++-common/musttail1.c: New test.
	* c-c++-common/musttail12.c: New test.
	* c-c++-common/musttail13.c: New test.
	* c-c++-common/musttail2.c: New test.
	* c-c++-common/musttail3.c: New test.
	* c-c++-common/musttail4.c: New test.
	* c-c++-common/musttail5.c: New test.
	* c-c++-common/musttail7.c: New test.
	* c-c++-common/musttail8.c: New test.
	* g++.dg/musttail10.C: New test.
	* g++.dg/musttail11.C: New test.
	* g++.dg/musttail6.C: New test.
	* g++.dg/musttail9.C: New test.