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/kern/linux/linux.cc

@@ -73,7 +73,7 @@ Linux::openSpecialFile(std::string path, Process *process,
     if (matched) {
         FILE *f = tmpfile();
         int fd = fileno(f);
-        size_t ret M5_VAR_USED = fwrite(data.c_str(), 1, data.size(), f);
+        M5_VAR_USED size_t ret = fwrite(data.c_str(), 1, data.size(), f);
         assert(ret == data.size());
         rewind(f);
         return fd;