support/check-host-rpath: remove trailing '/' in host dir

Despite the comment saying so, the trailing '/' in the host directory is
not removed. Note however that it is properly removed from extracted
RPATH tags.

This is not visible when the host directory is our default $(O)/host
location, but breaks for user-supplied external host directory, when
the user leaves a trailing slash in the path.

Fix that.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Peter Korsgaard <jacmet@uclibc.org>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
(cherry picked from commit f42e2621)