gcc: fix missing dependencies for selftests

Building GCC with multiple make jobs appears to trigger a race condition.  The build fails with:

/bin/bash: TOPDIR/tmp/work/x86_64-linux/gcc-cross-i686/9.3.0-r0/gcc-9.3.0/build.x86_64-linux.i686-poky-linux/./gcc/xgcc: No such file or directory

Signed-off-by: Steve Sakoman <steve@sakoman.com>