systemc: Update the gem5-within-systemc README to mention USE_SYSTEMC.
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 <2d08bb393b588a7f78bde56e4f99e16f4aa802a1@eit.uni-kl.de>
Maintainer: Gabe Black <5c8aa1266c03186393d9e7b3b4e3b14239af8201@google.com>

TUD-OS/gem5-dtu,TUD-OS/gem5-dtu,TUD-OS/gem5-dtu,gem5/gem5,TUD-OS/gem5-dtu,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,TUD-OS/gem5-dtu,TUD-OS/gem5-dtu,gem5/gem5,TUD-OS/gem5-dtu