sox: disable SSP when using MIPS Codescape toolchains

MIPS Codescape toolchains don't support stack-smashing protection
despite of using glibc, therefore we see failures like this one:

mips-img-linux-gnu/bin/ld: cannot find -lssp

Fixes:
  http://autobuild.buildroot.net/results/957/95721f7b88c46a20202fb02e408817097df965c3/



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