Introduce platform-agnostic ALWAYS_INLINE macro

`<attributes.h>` has been included in anticipation of the following
commit.

src/attributes.h

@@ -16,4 +16,12 @@
 #  define LIFETIMEBOUND
 #endif
 
+#if defined(__GNUC__)
+#  define ALWAYS_INLINE inline __attribute__((always_inline))
+#elif defined(_MSC_VER)
+#  define ALWAYS_INLINE __forceinline
+#else
+#  error No known always_inline attribute for this platform.
+#endif
+
 #endif // BITCOIN_ATTRIBUTES_H