xtensa: Add support for sibling call optimization

xtensa: Add support for sibling call optimization

This patch introduces support for sibling call optimization, when call0
ABI is in effect.

gcc/ChangeLog:

	* config/xtensa/xtensa-protos.h (xtensa_prepare_expand_call,
	xtensa_emit_sibcall): New prototypes.
	(xtensa_expand_epilogue): Add new argument that specifies whether
	or not sibling call.
	* config/xtensa/xtensa.cc (TARGET_FUNCTION_OK_FOR_SIBCALL):
	New macro definition.
	(xtensa_prepare_expand_call): New function in order to share
	the common code.
	(xtensa_emit_sibcall, xtensa_function_ok_for_sibcall):
	New functions.
	(xtensa_expand_epilogue): Add new argument sibcall_p and use it
	for sibling call handling.
	* config/xtensa/xtensa.md (call, call_value):
	Use xtensa_prepare_expand_call.
	(call_internal, call_value_internal):
	Add the condition in order to be disabled if sibling call.
	(sibcall, sibcall_value, sibcall_epilogue): New expansions.
	(sibcall_internal, sibcall_value_internal): New insn patterns,
	and split ones in order to take care of the indirect sibcalls.

gcc/testsuite/ChangeLog:

	* gcc.target/xtensa/sibcalls.c: New.