We already use relative links for other gcc libexec links, this changes the sdk
do_install to match elsewhere and use relative symlinks too. This makes things
slightly easier in the SDK installation process and standardises.
(From OE-Core rev: ecfa1141e731224cc5a099f8dfd22878f23359ec)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>