Introduce src.conf knob to build userland with retpoline

MFC r339511: Introduce src.conf knob to build userland with retpoline

WITH_RETPOLINE enables -mretpoline vulnerability mitigation in userland
for CVE-2017-5715.

MFC r340099: libcompat: disable retpoline when building build tools

These are built with the host toolchain which may not support retpoline.
While here, move the MK_ overrides to a separate line and sort them
alphabetically to support future changes.

MFC r340650: Avoid retpolineplt with static linking

Statically linked binaries linked with -zretpolineplt crash at startup
as lld produces a broken PLT.

MFC r340652: rescue: set NO_SHARED in Makefile

The rescue binary is built statically via the Makefile generated by
crunchgen, but that does not trigger other shared/static logic in
bsd.prog.mk - in particular

PR: 233336
Reported by: Peter Malcom (r339511), Charlie Li (r340652)
Approved by: re (gjb, early MFC)
Sponsored by: The FreeBSD Foundation