See the freedesktop bug: https://bugs.freedesktop.org/show_bug.cgi?id=35268 Because the initial-exec model, unsupported in musl, is still used by mesa we get segmentation faults in applications when attempting to create glx contexts. Until this is fixed, we should mask the nptl use flag for x11-base/xorg-server and media-libs/mesa under musl profiles.
https://github.com/gentoo/gentoo/pull/1042