Add compile-time platform check for JIT support

In issue #324, bus errors are reported when MoarVM is compiled
in a universal binary and runs on a 32 bit platform (OS X).
This adds a compile-time check for the platform, in the assumption
that the compiler will generate different code for each 'instance',
in which case it should disable the JIT support on 32 bit platforms.

src/jit/emit_x64.dasc

@@ -61,7 +61,15 @@
 
 
 const MVMint32 MVM_jit_support(void) {
+#ifdef __i386__
+    /* Usually, this file should only be compiled only on a amd64
+       platform; but when compiling 'fat' or 'universal' binaries, we
+       may compile it for other platform. In this case we use the
+       runtime check to disable the JIT */
+    return 0;
+#else
     return 1;
+#endif
 }
 
 const unsigned char * MVM_jit_actions(void) {