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.

tests/meson.build

@@ -15,7 +15,7 @@ cases = [
 test_driver = executable('test_driver', 'driver.c',
                          dependencies: libx86decode,
                          c_args: ['-D_GNU_SOURCE'])
-test_args = files('common.sh') + [test_driver.full_path(), '@0@'.format(pointer_size * 8)]
+test_args = files('common.sh') + [test_driver.full_path(), get_option('archmode')]
 
 foreach case : cases
   test(case[0], sh, args: test_args + files(case[1]))