scons: Disable partial linking on Mac OS

scons: Disable partial linking on Mac OS

Up until Apple LLVM version 10.0.0 (clang-1000.11.45.5), partial
linked objects do not expose symbols that are marked with the hidden
visibility and consequently building gem5 on Mac OS fails. As a
workaround, we disable partial linking, however, we may want to
revisit in the future.

Change-Id: I0a26dae082bf723c2bd49d90e4497e44ecab9c41
Signed-off-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/15235
Reviewed-by: Andrea Mondelli <andrea.mondelli@ucf.edu>
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>