toolchain.eclass: disallow USE="ssp -libssp" on mingw targets

toolchain.eclass: disallow USE="ssp -libssp" on mingw targets

gcc-8 added --disable-libssp flag. It used to be gentoo-specific
flag up to gcc-5.4.0 and gentoo was passing --disable-libssp on
all targets except solaris.

On gcc-6 gentoo stopped providing --disable-libssp flag (it became
no-op). USE=ssp became the default with gcc-6.

gcc-8 reintroduced --disable-libssp and that broke mingw. mingw
libc does not provide stack_check_*() functions and USE=ssp forces
compiler to emit those calls.

This change disallows disabling libssp on mingw if USE=ssp is set.

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>