gentoo: support running tests under sandbox

when glibc runs its tests, it does so by invoking the local library loader.
in Gentoo, we build/run inside of our "sandbox" which itself is linked against
libdl (so that it can load libraries and pull out symbols).  the trouble
is that when you upgrade from an older glibc to the new one, often times
internal symbols change name or abi.  this is normally OK as you cannot use
libc.so from say version 2.3.6 but libpthread.so from say version 2.5, so
we always say "keep all of the glibc libraries from the same build".  but
when glibc runs its tests, it uses dynamic paths to point to its new local
copies of libraries.  if the test doesnt use libdl, then glibc doesnt add
its path, and when sandbox triggers the loading of libdl, glibc does so
from the host system system.  this gets us into the case of all libraries
are from the locally compiled version of glibc except for libdl.so.

http://bugs.gentoo.org/56898