Define BIONIC_STOP_UNWIND for mips

This fixes mips and mips64 build failures.

Test: successful aosp_mips-eng and aosp_mips64-eng builds
Change-Id: I6648b9cae876cda13369cf10edf61ce9fdca342c
diff --git a/libc/private/bionic_macros.h b/libc/private/bionic_macros.h
index c0df917..a3a3ece 100644
--- a/libc/private/bionic_macros.h
+++ b/libc/private/bionic_macros.h
@@ -74,6 +74,8 @@
 #define BIONIC_STOP_UNWIND asm volatile(".cfi_undefined \%eip")
 #elif defined(__x86_64__)
 #define BIONIC_STOP_UNWIND asm volatile(".cfi_undefined \%rip")
+#elif defined (__mips__)
+#define BIONIC_STOP_UNWIND asm volatile(".cfi_undefined $ra")
 #endif
 
 #endif // _BIONIC_MACROS_H_