alpha: makefile: don't enforce small data model for kernel builds

Due to all of the goodness being packed into today's kernels, the
resulting image isn't as slim as it once was.

In light of this, don't pass -msmall-data to gcc, which otherwise results
in link failures due to impossible relocations when compiling anything but
the most trivial configurations.

Cc: Richard Henderson <rth@twiddle.net>
Cc: Ivan Kokshaysky <ink@jurassic.park.msu.ru>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Tested-by: Thorsten Kranzkowski <dl8bcu@dl8bcu.de>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Michael Cree <mcree@orcon.net.nz>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>