Remove now-untrue XXX comment that GCC is required

As of r256030 in-tree clang supports __attribute__((ms_abi)).
Merge r255944 (bdrewery) from HEAD i386:

  Use MK_SSP=no after including bsd.own.mk to disable SSP instead of
  user-knob WITH[OUT]_SSP to avoid hitting an error if user has WITH_SSP
  in their make.conf. Ports now use this knob.

    make[7]: "/usr/src/share/mk/bsd.own.mk" line 466: WITH_SSP and
    WITHOUT_SSP can't both be set.

    This is similar to previous cleanup done in r188895

This change in HEAD was made after the efi directory moved to amd64 on
the projects/efi branch.

Sponsored by:	The FreeBSD Foundation
Revert removal of i386 boot2

A merge from HEAD commit (r255961) also removed the build of the
sys/boot/i386/boot2 subdirectory.  I suspect this was a local change that
was accidentally included.

Sponsored by:	The FreeBSD Foundation