luajit: Disable for MIPS architecture with soft-float

Currently luajit is not supported on MIPS soft-float userlands. Upstream
plans to support it in the future:

   http://wiki.luajit.org/Open-Sponsorships#MIPS-Soft-Float-and-Dual-Number-Port

Fixes:

   http://autobuild.buildroot.net/results/d52/d5232a215e9e9aeb40b2e4e353329a27603c79ab/

Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>