systemc: Update the gem5-within-systemc README to mention USE_SYSTEMC.

When linking with an external systemc host, the native gem5 systemc API
should be disabled to avoid linking conflicts.

Change-Id: If7f9b3718fcdbdab5ebc9ef52e746f6bf9f2fb9c
Reviewed-on: https://gem5-review.googlesource.com/c/14520
Reviewed-by: Matthias Jung <jungma@eit.uni-kl.de>
Maintainer: Gabe Black <gabeblack@google.com>

util: Move the existing systemc example into a subdirectory.

This example is for how to embed gem5 within systemc and make it use
the systemc event queue. This used to be the only method of using
gem5 and systemc together, but now that there are other options, it's
ambiguous to have it as the only thing in the util/systemc directory.

This change moves it into a gem5_within_systemc subdirectory which
clearly shows what type of integration that example corresponds with.

Change-Id: I426d68ccb618397d820bef492cbb1ff8ef4a979b
Reviewed-on: https://gem5-review.googlesource.com/c/13375
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>