alpha: Notice if we're being run under QEMU

When building a generic kernel, do a run-time check on the serial
number, like we do for MILO.  When building a custom kernel, make
this a configure-time check.

Signed-off-by: Richard Henderson <rth@twiddle.net>

diff --cc arch/alpha/Kconfig
Simple merge

diff --cc arch/alpha/include/asm/machvec.h
Simple merge

diff --cc arch/alpha/kernel/setup.c
Simple merge