base: Simplify the definition of DTRACE.
base: Simplify the definition of DTRACE.

Instead of gating the behavior of DTRACE based on TRACING_ON in the
preprocessor, move it to C++. Beyond being a little simpler, this
ensures that the value of Debug::x is always valid (the proper header is
included, x is spelled correctly, etc) even if TRACING_ON is false.

Change-Id: Ie0085c0f8753ad5283ef1850d493706b977c21a8
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/45006
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Daniel Carvalho <91779fc734d358d9575ec4910de29bfcbcaf1b22@yahoo.com.br>
Maintainer: Jason Lowe-Power <92de577f37050ad05b60626d2f17913f40dcb1a2@gmail.com>

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