I've noticed the problem when tried to install
cross-compiler using following configuration:

    $ ./configure --target=s390x-unknown-linux-gnu
    make install Stage1Only=YES

Instead of expected tool prefix
    's390x-unknown-linux-gnu-'
Result was:
    's390x-ibm-linux-gnu-'

It's problematic as installed binaries appear in
unpredictable location.

The problem is caused by use of ${target} autoconf variable.
${target} contains a canocalized triplet.

Luckily we already have non-canonucalized target triplet
in ${TargetPlatformFull} variable. The change uses that
instead.

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>