Implement no_stack_protector attribute.

Implement no_stack_protector attribute.

gcc/ChangeLog:

2020-05-18  Martin Liska  <mliska@suse.cz>

	PR c/94722
	* cfgexpand.c (stack_protect_decl_phase):
	Guard with lookup_attribute("no_stack_protector") at
	various places.
	(expand_used_vars): Likewise here.
	* doc/extend.texi: Document no_stack_protector attribute.

gcc/ada/ChangeLog:

2020-05-18  Martin Liska  <mliska@suse.cz>

	PR c/94722
	* gcc-interface/utils.c (handle_no_stack_protect_attribute):
	New.
	(handle_stack_protect_attribute): Add error message for a
	no_stack_protector function.

gcc/c-family/ChangeLog:

2020-05-18  Martin Liska  <mliska@suse.cz>

	PR c/94722
	* c-attribs.c (handle_no_stack_protect_function_attribute): New.
	(handle_stack_protect_attribute): Add error message for a
	no_stack_protector function.

gcc/testsuite/ChangeLog:

2020-05-18  Martin Liska  <mliska@suse.cz>

	PR c/94722
	* g++.dg/no-stack-protector-attr-2.C: New test.
	* g++.dg/no-stack-protector-attr-3.C: New test.
	* g++.dg/no-stack-protector-attr.C: New test.