PR116080: Fix test suite checks for musttail

This is a new attempt to fix PR116080. The previous try was reverted
because it just broke a bunch of tests, hiding the problem.

- musttail behaves differently than tailcall at -O0. Some of the test
run at -O0, so add separate effective target tests for musttail.
- New effective target tests need to use unique file names
to make dejagnu caching work
- Change the tests to use new targets
- Add a external_musttail test to check for target's ability
to do tail calls between translation units. This covers some powerpc
ABIs.

gcc/testsuite/ChangeLog:

	PR testsuite/116080
	* c-c++-common/musttail1.c: Use musttail target.
	* c-c++-common/musttail12.c: Use struct_musttail target.
	* c-c++-common/musttail2.c: Use musttail target.
	* c-c++-common/musttail3.c: Likewise.
	* c-c++-common/musttail4.c: Likewise.
	* c-c++-common/musttail7.c: Likewise.
	* c-c++-common/musttail8.c: Likewise.
	* g++.dg/musttail10.C: Likewise. Replace powerpc checks with
	external_musttail.
	* g++.dg/musttail11.C: Use musttail target.
	* g++.dg/musttail6.C: Use musttail target. Replace powerpc
	checks with external_musttail.
	* g++.dg/musttail9.C: Use musttail target.
	* lib/target-supports.exp: Add musttail, struct_musttail,
	external_musttail targets. Remove optimization for musttail.
	Use unique file names for musttail.