scons: Remove mention of ALPHA from the SConstruct.

scons: Remove mention of ALPHA from the SConstruct.

I've arbitrarily chosen to make ARM the default ISA for now, since I
think it's the best supported ISA with X86 somewhere a little behind
it. As a compromise, I change all mention of ALPHA (or even ALPHA_SE!)
in comments to be X86 instead, so it gets some attention too.

Change-Id: I1d8edc7925ca2d94f11b26e2c0b9314216e9b97d
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/25463
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>