Add a target hook for sibcall epilogues

Add a target hook for sibcall epilogues

Epilogues for sibling calls are generated using the
sibcall_epilogue pattern.  One disadvantage of this approach
is that the target doesn't know which call the epilogue is for,
even though the code that generates the pattern has the call
to hand.

Although call instructions are currently rtxes, and so could be
passed as an operand to the pattern, the main point of introducing
rtx_insn was to move towards separating the rtx and insn types
(a good thing IMO).  There also isn't an existing practice of
passing genuine instructions (as opposed to labels) to
instruction patterns.

This patch therefore adds a hook that can be defined as an
alternative to sibcall_epilogue.  The advantage is that it
can be passed the call; the disadvantage is that it can't
use .md conveniences like generating instructions from
textual patterns (although most epilogues are too complex
to benefit much from that anyway).

gcc/
	* doc/tm.texi.in: Add TARGET_EMIT_EPILOGUE_FOR_SIBCALL.
	* doc/tm.texi: Regenerate.
	* target.def (emit_epilogue_for_sibcall): New hook.
	* calls.cc (can_implement_as_sibling_call_p): Use it.
	* function.cc (thread_prologue_and_epilogue_insns): Likewise.
	(reposition_prologue_and_epilogue_notes): Likewise.
	* config/aarch64/aarch64-protos.h (aarch64_expand_epilogue): Take
	an rtx_call_insn * rather than a bool.
	* config/aarch64/aarch64.cc (aarch64_expand_epilogue): Likewise.
	(TARGET_EMIT_EPILOGUE_FOR_SIBCALL): Define.
	* config/aarch64/aarch64.md (epilogue): Update call.
	(sibcall_epilogue): Delete.