openssl: use getrandom(2) instead of probing for getentropy(2)

The probing for getentropy(2) relies on re-declaring getentropy(2)
as weak and checking the address, but this is incompatible with
the _FORTIFY_SOURCE symbol renaming scheme. It's always present on
all supported FreeBSD versions now so we could cut it down to
unconditional use, but there's another segment for getrandom(2)
already that's cleaner to just add us to.

We should upstream this.

Reviewed by: kib (earlier version), markj
Sponsored by: Klara, Inc.
Sponsored by: Stormshield
Differential Revision: https://reviews.freebsd.org/D45976