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/o3/commit_impl.hh

@@ -701,7 +701,7 @@ DefaultCommit<Impl>::tick()
             // will be active.
             _nextStatus = Active;
 
-            const DynInstPtr &inst M5_VAR_USED = rob->readHeadInst(tid);
+            M5_VAR_USED const DynInstPtr &inst = rob->readHeadInst(tid);
 
             DPRINTF(Commit,"[tid:%i] Instruction [sn:%llu] PC %s is head of"
                     " ROB and ready to commit\n",