gcc: Fix inconsistent noexcept specifier for valarray in libstdc++

Backport of gcc upstream commit 2b2d97fc545635a0f6aa9c9ee3b017394bc494bf
to gcc release 9.5.0. This fix is available to all release-branches
except releases/gcc-9 because upstream do not support gcc-9 now.

Signed-off-by: Virendra Thakur <virendrak@kpit.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>