The problem is that I already have an "nptl only system", and on the
other hand, nptl supposed to be superior. (At the same time I admit
that I very likely don't see too much of the advantages of nptl on my
machine.) One could also build a system that supports both threading
libraries (as many distributions do).
The problem is, that nptl is "officially" not supported by uml yet. If
you also have the old linuxthreads libraries in your system, then
according to some mails I found on the net you simply have to
rename/remove the nptl ones in your uml drive to force the binaries to
use the obsolete ones. (Since I use Linux From Scratch it was not an
option for me.)
There was a post from one of the LFS authors about the changes to do to fix
this (it's just changing a couple of options when compiling glibc).