x86: unify crashkernel reservation for 32 and 64 bit

This patch moves the reserve_crashkernel() to setup.c and removes the
architecture-specific version. Both versions were more or less the same.

I tested it on both x86-64 and i386, with CONFIG_KEXEC on and off (so
that it compiles).

Signed-off-by: Bernhard Walle <bwalle@suse.de>
Cc: yhlu.kernel@gmail.com
Signed-off-by: Ingo Molnar <mingo@elte.hu>