unix: Allow to disable MICROPY_EMIT_X64 from commandline.

emitnative in particular requires nlr_* to be real functions, so doesn't
compile with MICROPY_NLR_SETJMP=1.

unix/mpconfigport.h

@@ -27,7 +27,9 @@
 // options to control how Micro Python is built
 
 #define MICROPY_ALLOC_PATH_MAX      (PATH_MAX)
+#ifndef MICROPY_EMIT_X64
 #define MICROPY_EMIT_X64            (1)
+#endif
 #define MICROPY_EMIT_THUMB          (0)
 #define MICROPY_EMIT_INLINE_THUMB   (0)
 #define MICROPY_ENABLE_GC           (1)