Add a target option for selecting a DWARF version

Certain platforms need to limit the DWARF version emitted (oxs, *bsd). This
change adds a dwarf_version entry to the options that allows a platform to
specify the dwarf version to use. By default this option is none and the default
DWARF version is selected.

Also adds an option for printing Option<u32> json keys

compiler/rustc_target/src/spec/openbsd_base.rs

@@ -26,6 +26,7 @@ pub fn opts() -> TargetOptions {
         position_independent_executables: true,
         eliminate_frame_pointer: false, // FIXME 43575
         relro_level: RelroLevel::Full,
+        dwarf_version: Some(2),
         ..Default::default()
     }
 }