Bug 31448: Disable firefox debug-symbols for linux-i686

This allows us to switch back to ld.gold as the linker, and fix #31618.

ld.gold fails when we build with debug symbols for linux-i686:

  30:31.20 toolkit/library/symverscript.stub
  30:31.26 toolkit/library/libxul.so
  30:39.58 /var/tmp/dist/binutils/bin/ld.gold.real: internal error in relocate_section, at i386.cc:3684