Make cpp.py less braindead.

The old code ignored operator precedence (!), despite having two tables
of operator precedence. The code's still pretty awful, but I've cleaned
it up enough to fix this, the most important bug.

This patch lets us correctly clean the uapi unistd.h, stat.h, and swab.h files,
and also fixes the mess we were already making of various old kernel
header files. I've added a bunch more tests, fixed the existing tests that
the existing script was already failing (!), and changed the script so that
the tests are run every time the script is run.

We can probably remove some of the old kernel header files that we were
parsing incorrectly, but we can worry about that later.

Bug: 11253477
Change-Id: Ie66c65b3a7ae13b4e98ed8038a6a534f06eae0e5

Conflicts:
	libc/kernel/uapi/asm-generic/stat.h
	libc/kernel/uapi/asm-generic/swab.h
	libc/kernel/uapi/asm-generic/unistd.h
	libc/kernel/uapi/linux/v4l2-dv-timings.h