scons: Trim down a check for mac OS and arch setting flags.

scons: Trim down a check for mac OS and arch setting flags.

This check used uname to determine if scons was running on macos, and
then a fairly elaborate check to see if the version was above 9, and if
the hardware supported 64 bit. I think at this point it's safe to assume
both that we're at least at macos 10 which is 19 years old, and that Mac
hardware supports 64 bit.

Change-Id: Ice66df2530bbcc929d3a37e7679634b75ba7b860
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/40857
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>