Removing absolute rpaths breaks multi-host builds

[brson]

http://buildbot.rust-lang.org/builders/master-mac/builds/94/steps/compile/logs/stdio
http://buildbot.rust-lang.org/builders/master-linux/builds/95/steps/compile/logs/stdio

These are configured something like ../configure --host-triples=x86_64-apple-darwin,i686-apple-darwin

[brson]

I am not sure why this happens off hand, but when bootstrapping the non-build-triple compilers artifacts are copied from e.g. build/x86_64-unknown-linux-gnu/ to build/i686-unknown-linux-gnu/.