aarch64: Add compiler support for Shadow Call Stack

Shadow Call Stack can be used to protect the return address of a
function at runtime, and clang already supports this feature[1].

To enable SCS in user mode, in addition to compiler, other support
is also required (as discussed in [2]). This patch only adds basic
support for SCS from the compiler side, and provides convenience
for users to enable SCS.

For linux kernel, only the support of the compiler is required.

[1] https://clang.llvm.org/docs/ShadowCallStack.html
[2] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102768



Signed-off-by: Dan Li <ashimida@linux.alibaba.com>

gcc/ChangeLog:

	* config/aarch64/aarch64.cc (SLOT_REQUIRED):
	Change wb_candidate[12] to wb_push_candidate[12].
	(aarch64_layout_frame): Likewise, and
	change callee_adjust when scs is enabled.
	(aarch64_save_callee_saves):
	Change wb_candidate[12] to wb_push_candidate[12].
	(aarch64_restore_callee_saves):
	Change wb_candidate[12] to wb_pop_candidate[12].
	(aarch64_get_separate_components):
	Change wb_candidate[12] to wb_push_candidate[12].
	(aarch64_expand_prologue): Push x30 onto SCS before it's
	pushed onto stack.
	(aarch64_expand_epilogue): Pop x30 frome SCS, while
	preventing it from being popped from the regular stack again.
	(aarch64_override_options_internal): Add SCS compile option check.
	(TARGET_HAVE_SHADOW_CALL_STACK): New hook.
	* config/aarch64/aarch64.h (struct GTY): Add is_scs_enabled,
	wb_pop_candidate[12], and rename wb_candidate[12] to
	wb_push_candidate[12].
	* config/aarch64/aarch64.md (scs_push): New template.
	(scs_pop): Likewise.
	* doc/invoke.texi: Document -fsanitize=shadow-call-stack.
	* doc/tm.texi: Regenerate.
	* doc/tm.texi.in: Add hook have_shadow_call_stack.
	* flag-types.h (enum sanitize_code):
	Add SANITIZE_SHADOW_CALL_STACK.
	* opts.cc (parse_sanitizer_options): Add shadow-call-stack
	and exclude SANITIZE_SHADOW_CALL_STACK.
	* target.def: New hook.
	* toplev.cc (process_options): Add SCS compile option check.
	* ubsan.cc (ubsan_expand_null_ifn): Enum type conversion.

gcc/testsuite/ChangeLog:

	* gcc.target/aarch64/shadow_call_stack_1.c: New test.
	* gcc.target/aarch64/shadow_call_stack_2.c: New test.
	* gcc.target/aarch64/shadow_call_stack_3.c: New test.
	* gcc.target/aarch64/shadow_call_stack_4.c: New test.
	* gcc.target/aarch64/shadow_call_stack_5.c: New test.
	* gcc.target/aarch64/shadow_call_stack_6.c: New test.
	* gcc.target/aarch64/shadow_call_stack_7.c: New test.
	* gcc.target/aarch64/shadow_call_stack_8.c: New test.