Block sigaltstack with ENOSYS by default

`absl::GetStackTrace` on Aarch64 since
bb7bbb12c7 will call sigaltstack.
`absl::Mutex` in debug mode uses `absl::GetStackTrace` causing many new syscall violations. An error in the sigaltstack should be tolerated, so this will fix the issue without opening up the policy too much.

PiperOrigin-RevId: 580885547
Change-Id: I1acf28bff0e2f6f236a262c0ca8fa74a6c57fada

sandboxed_api/sandbox2/policybuilder.cc

@@ -998,6 +998,8 @@ PolicyBuilder& PolicyBuilder::AllowStaticStartup() {
                                         JEQ32(PROT_READ, ALLOW),
                                     });
 
+  OverridableBlockSyscallWithErrno(__NR_sigaltstack, ENOSYS);
+
   return *this;
 }