Use libvpx defines to set name mangling rules

Change-Id: Ie5dbfe73bdb6132047f9ab7d5716714d50befea5

third_party/x86inc/README.libvpx

@@ -10,3 +10,5 @@ defines that help automatically allow assembly to work cross-platform.
 Local Modifications:
 Get configuration from vpx_config.asm.
 Prefix functions with vpx by default.
+Manage name mangling (prefixing with '_') manually because 'PREFIX' does not
+  exist in libvpx.

third_party/x86inc/x86inc.asm

@@ -75,6 +75,15 @@
     %define FORMAT_ELF 1
 %endif
 
+; Set PREFIX for libvpx builds.
+%if FORMAT_ELF
+    %undef PREFIX
+%elif WIN64
+    %undef PREFIX
+%else
+    %define PREFIX
+%endif
+
 %ifdef PREFIX
     %define mangle(x) _ %+ x
 %else