PR116080: Fix tail call dejagnu checks

PR116080: Fix tail call dejagnu checks

- Run the target_effective tail_call checks without optimization to
match the actual test cases.
- Add an extra check for external tail calls to handle targets like
powerpc that cannot tail call between different object files.
This one will also cover templates.

gcc/testsuite/ChangeLog:

	PR testsuite/116080
	* g++.dg/musttail10.C: Use external tail call target check.
	* g++.dg/musttail6.C: Dito.
	* lib/target-supports.exp: Add external_tail_call. Disable
	optimization for tail call checks.