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/kvm/x86_cpu.cc

@@ -68,7 +68,7 @@ using namespace X86ISA;
 // data) is used to indicate that a segment has been accessed.
 #define SEG_TYPE_BIT_ACCESSED 1
 
-struct FXSave
+struct M5_ATTR_PACKED FXSave
 {
     uint16_t fcw;
     uint16_t fsw;
@@ -97,7 +97,7 @@ struct FXSave
     uint8_t xmm[16][16];
 
     uint64_t reserved[12];
-} M5_ATTR_PACKED;
+};
 
 static_assert(sizeof(FXSave) == 512, "Unexpected size of FXSave");