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/cpu/simple/timing.cc

@@ -947,7 +947,7 @@ TimingSimpleCPU::completeDataAccess(PacketPtr pkt)
     // hardware transactional memory
 
     SimpleExecContext *t_info = threadInfo[curThread];
-    const bool is_htm_speculative M5_VAR_USED =
+    M5_VAR_USED const bool is_htm_speculative =
         t_info->inHtmTransactionalState();
 
     // received a response from the dcache: complete the load or store