Implement -Z function-sections=yes|no

This lets rustc users tweak whether all functions should be put in their own
TEXT section, using whatever default value the target defines if the flag
is missing.

compiler/rustc_codegen_llvm/src/back/write.rs

@@ -128,7 +128,8 @@ pub fn target_machine_factory(
     let (opt_level, _) = to_llvm_opt_settings(optlvl);
     let use_softfp = sess.opts.cg.soft_float;
 
-    let ffunction_sections = sess.target.options.function_sections;
+    let ffunction_sections =
+        sess.opts.debugging_opts.function_sections.unwrap_or(sess.target.options.function_sections);
     let fdata_sections = ffunction_sections;
 
     let code_model = to_llvm_code_model(sess.code_model());