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/net/sinic.cc

@@ -233,7 +233,7 @@ Device::read(PacketPtr pkt)
 
     prepareRead(cpu, index);
 
-    uint64_t value M5_VAR_USED = 0;
+    M5_VAR_USED uint64_t value = 0;
     if (pkt->getSize() == 4) {
         uint32_t reg = regData32(raddr);
         pkt->setLE(reg);

src/dev/net/tcp_iface.cc

@@ -253,7 +253,7 @@ TCPIface::connect()
 
 TCPIface::~TCPIface()
 {
-    int M5_VAR_USED ret;
+    M5_VAR_USED int ret;
 
     ret = close(sock);
     assert(ret == 0);