misc: Use compiler.hh macros when available.

Some places were hand coding __attribute__s when macros in compiler.hh
were available to do that job. Using the macros helps abstract away
compiler specific details and should be used when possible.

Change-Id: I94befebcfde2d673e874e9959588f69781bd9021
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/35975
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/arch/arm/isa/insts/sve.isa

@@ -2007,8 +2007,7 @@ let {{
         destPred.reset();
         for (unsigned i = 0; i < eCount; i++) {
             const Element& srcElem1 = AA64FpOp1_x[i];
-            %(src_elem_2_ty)s srcElem2 __attribute__((unused)) =
-                                                            %(src_elem_2)s;
+            M5_VAR_USED %(src_elem_2_ty)s srcElem2 = %(src_elem_2)s;
             bool destElem = false;
             if (tmpPred[i]) {
                 %(op)s

src/arch/x86/linux/linux.hh

@@ -243,7 +243,7 @@ class X86Linux32 : public X86Linux
 {
   public:
 
-    typedef struct {
+    typedef struct M5_ATTR_PACKED {
         uint64_t st_dev;
         uint8_t __pad0[4];
         uint32_t __st_ino;
@@ -263,7 +263,7 @@ class X86Linux32 : public X86Linux
         uint32_t st_ctimeX;
         uint32_t st_ctime_nsec;
         uint64_t st_ino;
-    } __attribute__((__packed__)) tgt_stat64;
+    } tgt_stat64;
 
     static const int TGT_SIGHUP         = 0x000001;
     static const int TGT_SIGINT         = 0x000002;