[libunwind] Tidy-up the testing configuration for libunwind

Start testing Apple backdeployment with older libunwinds, and stop
explicitly specifying the libunwind testing config, since it is
already selected correctly by default.

Differential Revision: https://reviews.llvm.org/D126470

NOKEYCHECK=True
GitOrigin-RevId: 92bbcfaa97408991813fda5c56e864c629f67a01
diff --git a/test/signal_frame.pass.cpp b/test/signal_frame.pass.cpp
index d9fb439..b17804e 100644
--- a/test/signal_frame.pass.cpp
+++ b/test/signal_frame.pass.cpp
@@ -9,8 +9,8 @@
 
 // Ensure that functions marked as signal frames are reported as such.
 
-// TODO: Investigate this failure on macOS
-// XFAIL: target={{.+}}-apple-darwin{{.+}}
+// TODO: Investigate this failure on Apple
+// XFAIL: target={{.+}}-apple-{{.+}}
 
 // TODO: Figure out why this fails with Memory Sanitizer.
 // XFAIL: msan