Allow specifying a different prefix in x86inc.asm

Currently the prefix is forced to vp9 for any function using
x86inc.asm.

Change-Id: Icbca57ce68a52e743bdd7e9be86cfe8353f274c1

third_party/x86inc/README.libvpx

@@ -9,3 +9,4 @@ defines that help automatically allow assembly to work cross-platform.
 
 Local Modifications:
 Some modifications to allow PIC to work with x86inc.
+Conditionally define program_name to allow overriding.

third_party/x86inc/x86inc.asm

@@ -36,7 +36,9 @@
 
 %include "vpx_config.asm"
 
+%ifndef program_name
 %define program_name vp9
+%endif
 
 
 %define UNIX64 0