build: add -mbranch-protection=bti to aarch64 hardening flags

This is a simpler (less hardening) version of #24123.
Scoped to aarch64 to avoid unused command line option warnings when
building on x86_64.

Related to #19075.

Author: fanquake

configure.ac

@@ -964,6 +964,11 @@ if test "$use_hardening" != "no"; then
       ;;
   esac
 
+  case $host in
+    *aarch64*)
+      AX_CHECK_COMPILE_FLAG([-mbranch-protection=bti], [HARDENED_CXXFLAGS="$HARDENED_CXXFLAGS -mbranch-protection=bti"])
+    ;;
+  esac
 
   dnl When enable_debug is yes, all optimizations are disabled.
   dnl However, FORTIFY_SOURCE requires that there is some level of optimization, otherwise it does nothing and just creates a compiler warning.