interp: ignore inline assembly in markExternal

The markExternal function is used when a global (function or global
variable) is somehow run at runtime. All the other globals it refers to
are from then on no longer known at compile time, so can't be used by
the interp package anymore.

This can also include inline assembly. While it is possible to modify
globals that way, it is only possible to modify exported globals:
similar to calling an undefined function (in C for example).

interp/testdata/basic.out.ll

@@ -26,6 +26,7 @@ entry:
   store i16 5, i16* @main.exposedValue1
   call void @modifyExternal(i32* bitcast (void ()* @willModifyGlobal to i32*))
   store i16 7, i16* @main.exposedValue2
+  call void @modifyExternal(i32* bitcast (void ()* @hasInlineAsm to i32*))
   call void @runtime.printint64(i64 6)
   call void @runtime.printint64(i64 -1)
   %agg = call { i8, i32, { float, { i64, i16 } } } @nestedStruct()
@@ -59,6 +60,12 @@ entry:
   ret void
 }
 
+define void @hasInlineAsm() {
+entry:
+  call void asm sideeffect "", ""()
+  ret void
+}
+
 define i64 @testSwitch(i64 %val) local_unnamed_addr {
 entry:
   switch i64 %val, label %otherwise [