misc: Update attribute syntax, and reorganize compiler.hh.

This change replaces the __attribute__ syntax with the now standard [[]]
syntax. It also reorganizes compiler.hh so that all special macros have
some explanatory text saying what they do, and each attribute which has a
standard version can use that if available and what version of c++ it's
standard in is put in a comment.

Also, the requirements as far as where you put [[]] style attributes are
a little more strict than the old school __attribute__ style. The use of
the attribute macros was updated to fit these new, more strict
requirements.

Change-Id: Iace44306a534111f1c38b9856dc9e88cd9b49d2a
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/35219
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/learning_gem5/part2/simple_cache.cc

@@ -230,7 +230,7 @@ SimpleCache::handleResponse(PacketPtr pkt)
         DPRINTF(SimpleCache, "Copying data from new packet to old\n");
         // We had to upgrade a previous packet. We can functionally deal with
         // the cache access now. It better be a hit.
-        bool hit M5_VAR_USED = accessFunctional(originalPacket);
+        M5_VAR_USED bool hit = accessFunctional(originalPacket);
         panic_if(!hit, "Should always hit after inserting");
         originalPacket->makeResponse();
         delete pkt; // We may need to delay this, I'm not sure.