icecc-create-env: Fix executable rpaths

Executables in the toolchain archive occasionally contain runtime
library search paths (RPATH) that use the $ORIGIN placeholder. However,
in order for that placeholder to work, /proc must be mounted. When
iceccd executes the toolchain in the chroot environment, it doesn't
mount /proc, so it is unable to resolve $ORIGIN resulting in a failure
to find dynamic libraries.

The fix is to replace $ORIGIN in executable RPATH entries with the known
chroot executable path. In order for this to work, the actual real path
to the executable must be resolved to remove any symlinks, otherwise the
calculate $ORIGIN replacement will be wrong. This is done by using
"readlink -f", which is an acceptable dependency because Yocto already
requires it.

Signed-off-by: Joshua Watt <JPEWhacker@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>