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/dev/arm/gic_v2.cc

@@ -389,7 +389,7 @@ GicV2::writeDistributor(PacketPtr pkt)
     const ContextID ctx = pkt->req->contextId();
     const size_t data_sz = pkt->getSize();
 
-    uint32_t pkt_data M5_VAR_USED;
+    M5_VAR_USED uint32_t pkt_data;
     switch (data_sz)
     {
       case 1:

src/dev/arm/smmu_v3_transl.hh

@@ -97,7 +97,7 @@ class SMMUTranslationProcess : public SMMUProcess
     TranslContext context;
 
     Tick recvTick;
-    Tick M5_CLASS_VAR_USED faultTick;
+    M5_CLASS_VAR_USED Tick faultTick;
 
     virtual void main(Yield &yield);