Fix macro redefinition for GNUC compilers > 6

GNUC compilers greater than version 6 already have the macro __always_inline defined. This fix checks GNUC version before defining the macro itself.

tmk_core/protocol/arm_atsam/usb/compiler.h

@@ -136,7 +136,7 @@
  */
 #if defined(__CC_ARM)
 #  define __always_inline             __forceinline
-#elif (defined __GNUC__)
+#elif (defined __GNUC__ && __GNUC__ <= 6)
 #  define __always_inline             __attribute__((__always_inline__))
 #elif (defined __ICCARM__)
 #  define __always_inline             _Pragma("inline=forced")