test-includes: Build a library instead of a binary

We need to build the .o's, but don't need to link. Build a static
library instead of a binary to accmoplish this. This removes the need to
have all the libc and crt stuff built, which is required for a
binary. In addition, trying to build a non-standard binary runs into
trouble with undefined symbols on arm related to EABI, even when using a
simplified startup with -nostdlib -e start.

Turn back on testing includes, now that it works.

Sponsored by: Netflix
Reviewed by: brooks, markj (prior version)
Differential Revision: https://reviews.freebsd.org/D33452