lang/gcc1[2-6]: actually fix build on powerpc64le/CURRENT

Without this patch, gcc attempts to enable support for IBM long double,
when IEEE long double is enabled, which is wrong. Allow to only
specify IEEE long double.

The whole patchset for IEEE long double on FreeBSD/powerpc64le
is being upstreamed at https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126012.