Add LABEL_MANGLE() to export label symbols from inside asm blocks.

Originally committed as revision 18934 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavutil/internal.h

@@ -99,6 +99,9 @@
 #    define offsetof(T,F) ((unsigned int)((char *)&((T *)0)->F))
 #endif
 
+/* Use to export labels from asm. */
+#define LABEL_MANGLE(a) EXTERN_PREFIX #a
+
 // Use rip-relative addressing if compiling PIC code on x86-64.
 #if ARCH_X86_64 && defined(PIC)
 #    define LOCAL_MANGLE(a) #a "(%%rip)"