glibc: re-enable stripping in cross builds

References to the host toolchain are leaking through debug symbols in
glibc, causing gnu cross-builds to always depend on the host toolchain.
The decision to not strip was made in 2012 in order to improve GNU/Hurd
support, and I suspect the reasons that justified it back then do not
apply anymore in 2019.

Closure size before:
/nix/store/v5pxj0bgg627hic2khk4d43z6cjp5v7d-hello-2.10-armv7l-unknown-linux-gnueabihf                            596.8M

After:
/nix/store/llp1ncmpar406rc2vhj7g5ix4yqwna3n-hello-2.10-armv7l-unknown-linux-gnueabihf     23.6M