Add __BIONIC_AVAILABILITY_GUARD().

Putting this behavior behind a macro so we can easily change it across
bionic if needed.

Bug: https://github.com/android/ndk/issues/2081
Test: None
Change-Id: Ia08b54a61b5b06f6fa4646f036ef1e788d7c3ece
diff --git a/libc/execinfo/include/execinfo.h b/libc/execinfo/include/execinfo.h
index e092c00..c8f9e21 100644
--- a/libc/execinfo/include/execinfo.h
+++ b/libc/execinfo/include/execinfo.h
@@ -30,8 +30,10 @@
 
 /*
  * This file is exported as part of libexecinfo for use with musl, which doesn't
- * define __INTRODUCED_IN.  Stub it out.
+ * define __INTRODUCED_IN or __BIONIC_AVAILABILITY_GUARD.  Stub them out.
  */
 #define __INTRODUCED_IN(x)
+#define __BIONIC_AVAILABILITY_GUARD(x) 1
 #include <bionic/execinfo.h>
+#undef __BIONIC_AVAILABILITY_GUARD
 #undef __INTRODUCED_IN