core-macros: fix defer unused variable for Clang 14 with Ubuntu 22.04.

Seen with Ubuntu 22.04 and Clang 14.0.0-1ubuntu1.1, it produces a
warning about the unused variable produced by the defer macro:
```
| tests/zchk.c:1101:13: error: unused variable '__defer__1103' [-Werror,-Wunused-variable,Unused Entity Issue]
|             defer({
|             ^
| ./lib-common/core/macros.h:875:29: note: expanded from macro 'defer'
|     _defer_with_name(_code, PFX_LINE(__defer__))
|                             ^
| ./lib-common/core/macros.h:668:24: note: expanded from macro 'PFX_LINE'
| #define PFX_LINE(pfx)  _PFX_LINE_SFX(pfx, __LINE__, )
|                        ^
| ./lib-common/core/macros.h:665:41: note: expanded from macro '_PFX_LINE_SFX'
| #define _PFX_LINE_SFX(pfx, line, sfx)   __PFX_LINE_SFX(pfx, line, sfx)
|                                         ^
| ./lib-common/core/macros.h:664:41: note: expanded from macro '__PFX_LINE_SFX'
| #define __PFX_LINE_SFX(pfx, line, sfx)  pfx##line##sfx
|                                         ^
| <scratch space>:166:1: note: expanded from here
| __defer__1103
```

Add `__attr_unused__` to avoid this warning.
Also add it for GCC to avoid any further compat issues.
It is just ignored for newer compilers anyway.

Change-Id: Ibf310812172d833d20190577793fe7df06f962bf
Priv-Id: fdcb58e1e7f2d63d2c80467fb3d2b18808400a57

Author: nicopauss

src/core/macros.h

@@ -868,7 +868,7 @@ static ALWAYS_INLINE void _defer_blk_cleanup(_defer_inner_b *defer_blk)
 }
 
 #define _defer_with_name(_code, _dname)                                      \
-    __attr_cleanup__(_defer_blk_cleanup)                                     \
+    __attr_cleanup__(_defer_blk_cleanup) __attr_unused__                     \
     void (^_dname)(void) = ^(void) { _code }                                 \
 
 #define defer(_code)                                                         \
@@ -881,9 +881,9 @@ static ALWAYS_INLINE void _defer_blk_cleanup(_defer_inner_b *defer_blk)
 #define __deferred
 
 #define _defer_with_name(_code, _dname_func, _dname_var)                     \
-    auto ALWAYS_INLINE void _dname_func(int *);                              \
-    __attr_cleanup__(_dname_func) int _dname_var;                            \
-    ALWAYS_INLINE void _dname_func(int *__defer_var)                         \
+    auto ALWAYS_INLINE __attr_unused__ void _dname_func(int *);              \
+    __attr_cleanup__(_dname_func) __attr_unused__ int _dname_var;            \
+    ALWAYS_INLINE __attr_unused__ void _dname_func(int *__defer_var)         \
     {                                                                        \
         (void)__defer_var;                                                   \
         { _code }                                                            \