package/gcc: use BR2_USE_WCHAR to set gcc libquadmath option

package/gcc: use BR2_USE_WCHAR to set gcc libquadmath option

BR2_TOOLCHAIN_BUILDROOT_WCHAR is only defined when uclibc is selected,
whereas BR2_USE_WCHAR is always defined. Due to this, we were disabling
quadmath support even with glibc or musl.

So, use BR2_USE_WCHAR to drive the gcc libquadmath option.

In addition, invert the logic of the condition to use positive logic,
and rework the comment to no longer mention gcc 4.6: libquadmath still
exists, and it still requires wchar support.

Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>