gcc: simplify musl patches for SSP support

Now that we are always explicitly passing gcc_cv_libc_provides_ssp,
there is no longer any reason to modify the gcc configure/configure.ac
to take into account the musl case. When a musl toolchain is being
built, BR2_TOOLCHAIN_HAS_SSP is always 'y', and therefore
gcc_cv_libc_provides_ssp=yes is always passed when building
gcc-initial and gcc-final.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>