Revert "vm_insnhelper.c: INLINE condition" [Bug #12316]

This reverts commit r54747.
!__clang__ is also essential.
Anyway clang inlines vm_getivar into both vm_call_ivar and
vm_getinstancevariable, which r54728 originally intended to.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@54751 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

vm_insnhelper.c

@@ -19,7 +19,7 @@
 /* control stack frame */
 
 #undef INLINE
-#if defined __GNUC__ && !defined __NO_INLINE__
+#if defined __GNUC__ && !defined __NO_INLINE__ && !defined __clang__
 #define INLINE inline
 #else
 #define INLINE static inline