Relative DT_RPATH/DT_RUNPATH vs. sysroot capable ld.
This fixes two problems. First, the --sysroot option wasn't available
with a ld configured without --with-sysroot, a historical accident.
This led to people configuring binutils with --with-sysroot=/ in order
to enable sysroot support, which exposes a case where ld wrongly
prepends the sysroot to a relative path.