specify the cpu type for LLVM for OpenBSD target

The initial purpose is to workaround the LLVM bug
https://llvm.org/bugs/show_bug.cgi?id=26554 for OpenBSD.

By default, the `cpu' is defined to `generic`. But with a 64bit
processor, the optimization for `generic` will use invalid asm code as
NOP (the generated code for NOP isn't a NOP).

According to rust-lang#20777, "x86-64" is the right thing to do for x86_64
builds.

Closes: rust-lang#31363

src/librustc_back/target/x86_64_unknown_openbsd.rs

@@ -12,6 +12,7 @@ use target::Target;
 
 pub fn target() -> Target {
     let mut base = super::openbsd_base::opts();
+    base.cpu = "x86-64".to_string();
     base.pre_link_args.push("-m64".to_string());
 
     Target {