toolchain: control vendor part in GNU_TARGET_NAME

This option allows to customize the "vendor" part of the
toolchain tuple, where the toolchain tuple has the form
<arch>-<vendor>-<os>-<libc>. Use this option in situations
where gcc might make different decisions based on the vendor
part of the tuple.

[Thomas: move the config option in a slightly different place, so that
it does not appear between the C library selection and the C library
options.]

Signed-off-by: "Noam Camus" <noamc@ezchip.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>