Add a libipchecksum that contains the C IP checksum code.

This is useful for test code that wants to statically link the IP
checksum code. This is not currently possible because libnetutils
is vendor_available and making it available for static linking is
discouraged.

Test: m libnetutils libipchecksum
Original change: https://android-review.googlesource.com/c/platform/system/core/+/1359262

Change-Id: I18fc67d096c7b27804db37621c6662f65d4a78c2
Merged-In: Ic7f1864902581b8a5e3626860565b78c168bdb7f
(cherry picked from commit d4c69114)