enable fp-elim when debug info is disabled

This can almost be fully disabled, as it no longer breaks retrieving a
backtrace on OS X as verified by @alexcrichton. However, it still
breaks retrieving the values of parameters. This should be fixable in
the future via a proper location list...

Closes #7477

src/rustllvm/PassWrapper.cpp

@@ -68,7 +68,8 @@ LLVMRustCreateTargetMachine(const char *triple,
                             Reloc::Model RM,
                             CodeGenOpt::Level OptLevel,
                             bool EnableSegmentedStacks,
-                            bool UseSoftFloat) {
+                            bool UseSoftFloat,
+                            bool NoFramePointerElim) {
     std::string Error;
     Triple Trip(Triple::normalize(triple));
     const llvm::Target *TheTarget = TargetRegistry::lookupTarget(Trip.getTriple(),
@@ -79,7 +80,7 @@ LLVMRustCreateTargetMachine(const char *triple,
     }
 
     TargetOptions Options;
-    Options.NoFramePointerElim = true;
+    Options.NoFramePointerElim = NoFramePointerElim;
     Options.EnableSegmentedStacks = EnableSegmentedStacks;
     Options.FloatABIType = FloatABI::Default;
     Options.UseSoftFloat = UseSoftFloat;