FreeBSD_HEAD_amd64_gcc - Build #1360 - Fixed

FreeBSD_HEAD_amd64_gcc - Build #1360 - Fixed:

Build information: https://jenkins.FreeBSD.org/job/FreeBSD_HEAD_amd64_gcc/1360/
Full change log: https://jenkins.FreeBSD.org/job/FreeBSD_HEAD_amd64_gcc/1360/changes
Full build log: https://jenkins.FreeBSD.org/job/FreeBSD_HEAD_amd64_gcc/1360/console

Change summaries:

302439 by cem:
iflib: Fix typo in 'iflib_rx_miss_bufs' sysctl name

It looks like these sysctls were copy-pasted from netmap.  Most were changed
from 'ixl_' prefix to 'iflib_', but this one was missed.

Fix the "can't re-use a leaf (ixl_rx_miss_bufs)!" warning.

Reported by:	dim_at_ and others
Sponsored by:	EMC / Isilon Storage Division

302438 by ngie:
Revert r302403

lang/gcc{48,49,5} lacks -fformat-extensions support (causing build errors, which
is what prompted r302403 to be committed). devel/amd64-gcc on the other hand
(which is used by Jenkins), has the support.

This fixes the Jenkins failure emails due to excessive warnings being produced
with "make buildkernel".

Discussed with: lwhsu
Reported by: Jenkins (FreeBSD_HEAD_amd64_gcc job)
Sponsored by: EMC / Isilon Storage Division