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/testers/memtest/memtest.cc

@@ -245,7 +245,7 @@ MemTest::tick()
     if (cmd < percentReads) {
         // start by ensuring there is a reference value if we have not
         // seen this address before
-        uint8_t M5_VAR_USED ref_data = 0;
+        M5_VAR_USED uint8_t ref_data = 0;
         auto ref = referenceData.find(req->getPaddr());
         if (ref == referenceData.end()) {
             referenceData[req->getPaddr()] = 0;