Makefile: Unexport MACHINE variable

Fixes bug 7448

Otherwise glibc build fails.

[Peter: reworded]
Signed-off-by: Risto Avila <risto.avila@digia.com>
Reviewed-by: Luca Ceresoli <luca@lucaceresoli.net>
Tested-by: Luca Ceresoli <luca@lucaceresoli.net>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>