haproxy: disable on sh4 with gcc < 7

Build fails on sh4:
src/dns.c:290:1: error: unable to find a register to spill in class 'R0_REGS'
 }

This build failure seems related to a known gcc bug that has been fixed
only in gcc 7:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=60040

Fixes:
 - http://autobuild.buildroot.org/results/2e181cc874d5389f10ecddb0d11253c3aa5e7fc4



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>