test-resume-sig-rt: test unw_resume() in presence of "realtime" signal frame

Introduce a new test case that is derived from test-resume-sig, but
using the SA_SIGINFO sigaction() flag. This case is referred in the
linux kernel sources as "realtime" signal handler, and is handled
differently in the kernel on many architectures and in libunwind as
well.
diff --git a/.gitignore b/.gitignore
index eee3921..f5fe6dc 100644
--- a/.gitignore
+++ b/.gitignore
@@ -36,6 +36,7 @@
 tests/[GL]test-exc
 tests/[GL]test-init
 tests/[GL]test-resume-sig
+tests/[GL]test-resume-sig-rt
 tests/[GL]perf-simple
 tests/Ltest-nomalloc
 tests/Ltest-nocalloc