i386: Add indirect_return function attribute

i386: Add indirect_return function attribute

On x86, swapcontext may return via indirect branch when shadow stack
is enabled.  To support code instrumentation of control-flow transfers
with -fcf-protection, add indirect_return function attribute to inform
compiler that a function may return via indirect branch.

Note: Unlike setjmp, swapcontext only returns once.  Mark it return
twice will unnecessarily disable compiler optimization as shown in
the testcase here.

gcc/

	PR target/85620
	* config/i386/i386.c (rest_of_insert_endbranch): Also generate
	ENDBRANCH for non-tail call which may return via indirect branch.
	* doc/extend.texi: Document indirect_return attribute.

gcc/testsuite/

	PR target/85620
	* gcc.target/i386/pr85620-1.c: New test.
	* gcc.target/i386/pr85620-2.c: Likewise.
	* gcc.target/i386/pr85620-3.c: Likewise.
	* gcc.target/i386/pr85620-4.c: Likewise.

From-SVN: r262370