[libunwind] Adjust the signal_frame test for Arm

Summary:
This patch adjusts the signal_frame.pass.cpp to pass on Arm targets:
* When Arm EHABI is used the unwinder does not use DWARF, hence the
  DWARF-specific check unw_is_signal_frame() must be disabled.
* Certain C libraries don't include EH tables, so the unwinder must
  not try to step out of main(). The patch moves the test code out of
  main() into a separate function to avoid this.

Reviewers: saugustine, ostannard, phosek, jfb, mclow.lists

Reviewed By: saugustine

Subscribers: dexonsmith, aprantl, kristof.beyls, christof, libcxx-commits, pbarrio, labrinea

Tags: #libc

Differential Revision: https://reviews.llvm.org/D70397

libunwind/test/signal_frame.pass.cpp

@@ -13,13 +13,19 @@
 #include <stdlib.h>
 #include <libunwind.h>
 
-int main(void) {
+void test() {
   asm(".cfi_signal_frame");
   unw_cursor_t cursor;
   unw_context_t uc;
   unw_getcontext(&uc);
   unw_init_local(&cursor, &uc);
   assert(unw_step(&cursor) > 0);
+#if !defined(_LIBUNWIND_ARM_EHABI)
   assert(unw_is_signal_frame(&cursor));
+#endif
+}
+
+int main() {
+  test();
   return 0;
 }