Add RELRO build knob, default to enabled

Note that lld enables relro by default, so that we already had either
partial or full RELRO, depending on the state of the BIND_NOW knob.

Add a RELRO knob so that the option can be disabled if desired, and so
that builds using the GNU toolchain are equivalent to those using the
standard Clang/LLVM toolchain.

Reviewed by: markj
MFC after: 3 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D35545