Pull up following revision(s) (requested by mrg in ticket #731):
sys/arch/i386/conf/files.i386: revision 1.392
sys/arch/i386/conf/GENERIC: revision 1.1175
sys/arch/i386/conf/GENERIC: revision 1.1176
sys/arch/amd64/conf/files.amd64: revision 1.102
sys/arch/i386/conf/GENERIC: revision 1.1177
share/man/man4/options.4: revision 1.485 (patch)
sys/arch/i386/conf/Makefile.i386: revision 1.190
sys/arch/amd64/conf/GENERIC: revision 1.487
sys/arch/amd64/conf/ALL: revision 1.85
sys/arch/amd64/conf/GENERIC: revision 1.488
sys/arch/amd64/conf/GENERIC: revision 1.489
sys/arch/amd64/conf/Makefile.amd64: revision 1.67
sys/arch/i386/conf/ALL: revision 1.437

add an SPECTRE_V2_GCC_MITIGATION option to x86 kernels, that turns
on the GCC spectre v2 mitigation options.
XXX: pullup-8.
XXX: turn on in all kernels.

actually do what the previous change said:
don't turn on the new 'SPECTRE_V2_GCC_MITIGATION' option yet.

turn on GCC spectre v2 mitigation options.
XXX: amd64 ALL doesn't build for me right now

(martin)