toolchain/gcc.h: Add macro to get address of unaligned member

In principle getting the address of an unaligned structure member
is undefined behaviour, and clang warns as much.
Let's provide a macro which can return such an address.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>

Author: aescolar

include/zephyr/toolchain/gcc.h

@@ -174,6 +174,16 @@ do {                                                                    \
 
 #endif
 
+/*
+ * Get the address of a structure member even if the member may not be properly
+ * aligned. Note that accessing such an address must be done with care (for
+ * example with UNALIGNED_GET/PUT) and cannot be in general de-referenced to
+ * access the member directly, as that would cause a fault in architectures
+ * which have alignment requirements.
+ */
+#define UNALIGNED_MEMBER_ADDR(_p, _member) ((__typeof__(_p->_member) *) \
+		(((intptr_t)(_p)) + offsetof(__typeof__(*_p), _member)))
+
 /* Double indirection to ensure section names are expanded before
  * stringification
  */