SELFOUR-404: Remove hardcoded constants in stub generator.

`syscall_stub_gen` has hardcoded constants for word size, this patch removes
that constraint and makes the word size an argument to the program instead.
The word size can either be passed either as command line arugmemt (-w option)
or a path to the build configuration file can be given(-c option) and the
generator will use the value of `CONFIG_WORD_SIZE` instead.
The `Makefile` in `libsel4` calls the generator with the configuration file as
an option.

Ideally, the architecture should be determined from the configration file, but
since we don't have unit tests for the generator, I will make one change at a
time.

This commit also includes a few changes to make it Python 2.7 compliant.

libsel4/Makefile

@@ -113,4 +113,4 @@ include/interfaces/sel4_client.h: \
 	@${CHANGED_PATH} $@ \
         python ${SOURCE_DIR}/tools/syscall_stub_gen.py \
         --buffer \
-        -a $(SEL4_ARCH) -o $@ $^
+        -a $(SEL4_ARCH) -c ${srctree}/.config -o $@ $^