gcc: fixed parse-config to only inject -fstack-protector if we have a proper compiler supporting it

Note:

At stage 0 (toolchain) we are building a static gcc with explicitely disabled
stack-smashing protection.
uclibc: fixed to enable __progname support if GCC stack-smashing protection is enabled
busybox: change build priority (102.001 -> 107.000)

It doesn't make much sense to build busybox before building the compiler
when cross-compiling.