Pull up following revision(s) (requested by mrg in ticket #682):

tools/gcc/gcc-version.mk: revision 1.10 (via patch)
external/gpl3/gcc.old/dist/gcc/config/i386/i386.c: revision 1.10
external/gpl3/gcc.old/dist/gcc/config/i386/i386-protos.h: revision 1.6
external/gpl3/gcc.old/dist/gcc/config/i386/constraints.md: revision 1.6
external/gpl3/gcc.old/dist/gcc/doc/invoke.texi: revision 1.7
external/gpl3/gcc.old/dist/gcc/config/i386/i386.md: revision 1.7
external/gpl3/gcc.old/dist/gcc/doc/extend.texi: revision 1.7
external/gpl3/gcc.old/dist/gcc/config/i386/i386-opts.h: revision 1.4
external/gpl3/gcc.old/dist/gcc/config/i386/predicates.md: revision 1.6
external/gpl3/gcc.old/dist/gcc/config/i386/i386.h: revision 1.7
external/gpl3/gcc.old/dist/gcc/config/i386/i386.opt: revision 1.7
(with external/gpl3/gcc.old/ -> external/gpl3/gcc/)

add the spectre mitigation options for x86:

  -mindirect-branch=<choice>
  -mfunction-return=<choice>
  -mindirect-branch-register

the values for 'choice' are "keep" (default, existing behaviour),
"thunk", "thunk-inline", and "thunk-extern".

as taken from the Ubuntu port of these changes in their
ubuntu:gcc-5_5.5.0-8ubuntu1.diff.  i've also included the doc
updates that are missing from ubuntu from gcc itself.
i've tested both i386 and amd64 fairly heavily with these options
enabled in both kernels and userland, atf runs and hundreds of
package builds.

bump the NetBSD GCC version.  both GCC 5 and 6 got x86 spectre
mitigation code, and both are now "nb2 20180327".

(martin)