build_library: Clean up submit warnings.

Replace deprecated `getargspec` method with the suggested
`getfullargspec`.

BUG=None
TEST=./build_image --board=zork & flash and install image.

Change-Id: I7e6e6508b3463ff52d9c0845a19b9d53dd01b6fa
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosutils/+/2589757
Tested-by: Vyshu Khota <vyshu@google.com>
Commit-Queue: Vyshu Khota <vyshu@google.com>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Amin Hassani <ahassani@chromium.org>