Add compile-time option for architecture mode

This allows to decode x86-32 machine code on a 64-bit platform (but
not vice versa). As a side-effect, we also get rid of pointer-size
detection for architecture selection.

decode.h

@@ -5,6 +5,10 @@
 #include <stddef.h>
 #include <stdint.h>
 
+#if defined(ARCH_X86_64) && __SIZEOF_POINTER__ < 8
+#error "Decoding x86-64 requires a 64-bit architecture"
+#endif
+
 #ifndef ssize_t
 #define ssize_t intptr_t
 #endif