asm_*_offsets to define variables as constants

This change is to allow obj_int_extract to extract all integers
in the data segment. With the const keyword these variables are
forced into the .rodata segment even for zero variable value.

We had a problem before that zero valueed variables would get
assigned to BSS segment that fooled obj_int_extract to give
incorrect values.

Change-Id: Icd94f80a8ab356879894ca508bf132d20b865299

vpx_ports/asm_offsets.h

@@ -23,7 +23,7 @@
 #define BEGIN int main(void) {
 #define END return 0; }
 #else
-#define DEFINE(sym, val) int sym = val;
+#define DEFINE(sym, val) const int sym = val;
 #define BEGIN
 #define END
 #endif