package/gcc: enable float128 on powerpc64le with glibc >= 2.26 toolchains

package/gcc: enable float128 on powerpc64le with glibc >= 2.26 toolchains

float128 is now required on powerpc64le with glibc >= 2.26 toolchains.

Fixes:
running configure fragment for sysdeps/gnu
running configure fragment for sysdeps/powerpc/powerpc64le
checking if [...]powerpc64le-buildroot-linux-gnu-gcc supports binary128 floating point type... no
checking if the target machine is at least POWER8... yes
configure: error: ***  binary128 floating point type (GCC >= 6.2) is required on powerpc64le.

In order to prepare the glibc bump to this version, we enable
binary128 floating point in gcc. This require at least gcc >= 6.2.

Disable gcc 4.x and 5.x for internal toolchain.

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>