libgcc: aarch64: Add SME unwinder support

To support the ZA lazy save scheme, the PCS requires the unwinder to
reset the SME state to PSTATE.SM=0, PSTATE.ZA=0, TPIDR2_EL0=0 on entry
to an exception handler. We use the __arm_za_disable SME runtime call
unconditionally to achieve this.
https://github.com/ARM-software/abi-aa/blob/main/aapcs64/aapcs64.rst#exceptions

The hidden alias is used to avoid a PLT and avoid inconsistent VPCS
marking (we don't rely on special PCS at the call site). In case of
static linking the SME runtime init code is linked in code that raises
exceptions.

libgcc/ChangeLog:

	* config/aarch64/__arm_za_disable.S: Add hidden alias.
	* config/aarch64/aarch64-unwind.h: Reset the SME state before
	EH return via the _Unwind_Frames_Extra hook.

libgcc/config/aarch64/__arm_za_disable.S

@@ -63,3 +63,8 @@ ENTRY (__arm_za_disable)
 L(end):
 	ret
 END (__arm_za_disable)
+
+/* Hidden alias used by the unwinder.  */
+.global __libgcc_arm_za_disable
+.hidden __libgcc_arm_za_disable
+.set __libgcc_arm_za_disable, __arm_za_disable

libgcc/config/aarch64/aarch64-unwind.h

@@ -78,4 +78,20 @@ aarch64_demangle_return_addr (struct _Unwind_Context *context,
   return addr;
 }
 
+/* SME runtime function local to libgcc, streaming compatible
+   and preserves more registers than the base PCS requires, but
+   we don't rely on that here.  */
+__attribute__ ((visibility ("hidden")))
+void __libgcc_arm_za_disable (void);
+
+/* Disable the SME ZA state in case an unwound frame used the ZA
+   lazy saving scheme.  */
+#undef _Unwind_Frames_Extra
+#define _Unwind_Frames_Extra(x)				\
+  do							\
+    {							\
+      __libgcc_arm_za_disable ();			\
+    }							\
+  while (0)
+
 #endif /* defined AARCH64_UNWIND_H && defined __ILP32__ */