coroutines: Fix compile error with symmetric transfers [PR94359]

coroutines: Fix compile error with symmetric transfers [PR94359]

For symmetric transfers to work with C++20 coroutines, it is
currently necessary to tail call the callee coroutine from resume
method of the caller coroutine. The current codegen marks these
resume calls as "MUST_TAIL_CALL" to indicate that the tail call is
required for correctness.

Unfortunately, several targets have ABI constraints that prevent
an indirect tail-call, which results in the PRs compile error.

The change here tests the target sibcall hook for the resume
expression and only marks it as requiring a tail call if that's
supported.

This doesn't fix the underlying problem; that really a solution is
needed to allow the tail-calls (or equivalent) to take place - but
that will be deferred until next stage 1.

gcc/cp/ChangeLog:

2020-04-14  Iain Sandoe  <iain@sandoe.co.uk>

	PR c++/94359
	* coroutines.cc (build_actor_fn): Check that the target can
	support the resume tailcall before mandating it.

gcc/testsuite/ChangeLog:

2020-04-14  Iain Sandoe  <iain@sandoe.co.uk>

	PR c++/94359
	* g++.dg/coroutines/torture/symmetric-transfer-00-basic.C:
	Expect a run fail for targets without arbitrary indirect
	tail-calls.