sparc: Set UTS_MACHINE correctly.

"ARCH" can be just about anything, so we shouldn't end up
with UTS_MACHINE of "sparc" in a 64-bit kernel build just
because someone set the personality using 'sparc32' or
similar.  CONFIG_SPARC64 drives the compilation and
therefore provides the definitive value, not "ARCH".

This mirrors commit 8c6531f7
(x86: correctly set UTS_MACHINE for "make ARCH=x86")

Signed-off-by: David S. Miller <davem@davemloft.net>