libarchive: fix mismatch between library and test configuration

I was investigating libarchive test failures on CheriBSD and it turns out
we get a reproducible SIGBUS for test_archive_m5, etc. Debugging this shows
that libarchive and the tests disagree when it comes to the definition of
archive_md5_ctx: libarchive assumes it's the OpenSSL type whereas the test
use the libmd type. The latter is not necessarily aligned enough to store
a pointer (16 bytes for CHERI RISC-V), so we were crashing when storing
EVP_MD_CTX* to an 8-byte-aligned archive_md5_ctx.

To avoid problems like this in the future, factor out the common compiler
flags into a Makefile.inc and include that from the tests Makefile.

Reviewed By: lwhsu
Differential Revision: https://reviews.freebsd.org/D26469