gcc 4.8.x does not compile a working kernel for sparc

Since the switch to 4.8.x as default, the qemu-sparc target is broken.
For a gcc bug report see here:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=60624



Switch back to gcc 4.7.x as default for sparc.
Disable 4.8/4.9 as suggested by Thomas Petazzoni.
I even disabled gcc snapshot, it works right now, because
it is an old 4.8.0 snapshot by default, but as soon as this is updated
sparc build will break.

Signed-off-by: Waldemar Brodkorb <wbx@openadk.org>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>