Add a WITH_BIND_NOW build knob

The linker's -z now flag sets the DF_BIND_NOW flag, which signals to the
runtime loader that all relocation processing should be performed at
process startup rather than on demand. In combination with lld's
default of enabling relro this causes the GOT to be made read-only when
the process starts, preventing straightforward GOT overwrite attacks.

Shawn Webb discovered a failure on HardenedBSD with BIND_NOW and ifunc
use, which resulted in my rtld fix in r340137. Add a BIND_NOW knob as
it is trivial to do so and is a useful ELF hardening feature. This
change is equivalent to HardenedBSD's but not identical as there are
other diffs/conflicts nearby.

Note that our ELF Tool Chain readelf does not currently decode the
DF_BIND_NOW flag - see PR232983.

Reviewed by: brooks
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D17846