meson: Revert commit overriding C++ standard with gnu++11 on ppc64el

meson: Revert commit overriding C++ standard with gnu++11 on ppc64el

Since a few versions, mesa now needs c++14 and compiling with gnu++11
on ppc64el fails.
Let's use the default standard and fix the collision of types between
c++ and altivec in a another patch.

Cc: mesa-stable
Signed-off-by: Frédéric Bonnard <frediz@debian.org>
Reviewed-by: Francisco Jerez <currojerez@riseup.net>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4948>