scons: Disable systemc on Mac OS

scons: Disable systemc on Mac OS

LLVM on Mac OS up until version 10.0.0 doesn't seem to support the
attribute for weak linking and fails to compile. Additionally, even
the basic example tests fail. Therefore, this change disables systemc
by default on Mac OS until we address these issues.

Change-Id: Iaa6360ec61317e01f4e645af8b509961b3144282
Signed-off-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/14818
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>