Add a very minimal set of .cfi_* statements to get part of backtraces

working (on hello world at least):

~/inst/gdb/bin/gdb --args ./foo
(gdb) b write
...
(gdb) r
...
Breakpoint 1, 0xf7f04270 in write () from /lib32/libc.so.6
(gdb) bt
0  0xf7f04270 in write () from /lib32/libc.so.6
1  0x0804931a in rust_native_cdecl_3 ()
2  0x080487d7 in _rust_wrapper3_ ()
3  0x0804890a in _rust_fn5_main ()
4  0x08049440 in rust_native_cdecl_7 ()

src/rustllvm/RustWrapper.cpp

@@ -54,7 +54,7 @@ extern "C" void LLVMRustWriteOutputFile(LLVMPassManagerRef PMR, LLVMModuleRef M,
                                         LLVMCodeGenFileType FileType) {
 
   // Set compilation options.
-  llvm::NoFramePointerElim = true;
+  llvm::UnwindTablesMandatory = true;
 
   InitializeAllTargets();
   InitializeAllAsmPrinters();