I use the xbox profile with a 2.4 kernel (this kernel is currently hard masked, but was the only kernel who was ever marked stable for the Xbox).
Portage wants to upgrade glibc to sys-libs/glibc-2.4-r3 (from 2.3.6-r4), although this is not possible. After unpacking it says:
* glibc-2.4 is nptl-only!
!!! please add USE='nptl nptlonly' to make.conf
I think portage should prevent to emerge glibc-2.4, if a 2.4 kernel is installed.
I saw that there are no-ntpl profiles, should I switch to one of them or mask >=glibc-2.4?
Upgrade your kernel to a supported version or p.mask this glib version.
I think there is no supported version. All kernels are marked unstable. Would switching to a no-ntpl profile work?
The package.masked one is definitely totally unsupported. Already told you above what you should do. There's no no-nptl or 2.4 profile for Xbox.