Fix gcc -flto build, by adding LLVM_ATTRIBUTE_USED to

LLVMX86CompilationCallback2, called from inline asm.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@183824 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Target/X86/X86JITInfo.cpp

@@ -339,6 +339,7 @@ extern "C" {
 /// must locate the start of the stub or call site and pass it into the JIT
 /// compiler function.
 extern "C" {
+LLVM_ATTRIBUTE_USED // Referenced from inline asm.
 LLVM_LIBRARY_VISIBILITY void LLVMX86CompilationCallback2(intptr_t *StackPtr,
                                                          intptr_t RetAddr) {
   intptr_t *RetAddrLoc = &StackPtr[1];