zig cc: add -ffunction-sections integration

Also update to latest LLVM 13 command line options.

See #6290

src/main.zig

@@ -1290,6 +1290,8 @@ fn buildOutputType(
                     .no_red_zone => want_red_zone = false,
                     .omit_frame_pointer => omit_frame_pointer = true,
                     .no_omit_frame_pointer => omit_frame_pointer = false,
+                    .function_sections => function_sections = true,
+                    .no_function_sections => function_sections = false,
                     .unwind_tables => want_unwind_tables = true,
                     .no_unwind_tables => want_unwind_tables = false,
                     .nostdlib => ensure_libc_on_non_freestanding = false,
@@ -3821,6 +3823,8 @@ pub const ClangArgIterator = struct {
         strip,
         exec_model,
         emit_llvm,
+        function_sections,
+        no_function_sections,
     };
 
     const Args = struct {

tools/update_clang_options.zig

@@ -308,6 +308,14 @@ const known_options = [_]KnownOpt{
         .name = "fno-omit-frame-pointer",
         .ident = "no_omit_frame_pointer",
     },
+    .{
+        .name = "ffunction-sections",
+        .ident = "function_sections",
+    },
+    .{
+        .name = "fno-function-sections",
+        .ident = "no_function_sections",
+    },
     .{
         .name = "MD",
         .ident = "dep_file",