handle BOMs in linker response files

Decode UTF-16LE response files with a BOM and strip UTF-8 BOMs before
parsing arguments. Add coverage for UTF-16 response-file input.

src/linker/lnk_config.c

@@ -711,6 +711,18 @@ lnk_parse_export_directive(Arena *arena, String8 directive, LNK_Obj *obj, PE_Exp
   return is_parsed;
 }
 
+internal String8
+lnk_text_file_string_from_data(Arena *arena, String8 data)
+{
+  String8 result = data;
+  if (data.size >= 2 && data.str[0] == 0xff && data.str[1] == 0xfe) {
+    result = str8_from_16(arena, str16((U16 *)(data.str + 2), (data.size - 2) / sizeof(U16)));
+  } else if (data.size >= 3 && data.str[0] == 0xef && data.str[1] == 0xbb && data.str[2] == 0xbf) {
+    result = str8_skip(data, 3);
+  }
+  return result;
+}
+
 internal B32
 lnk_parse_merge_directive(String8 string, LNK_Obj *obj, LNK_MergeDirective *out)
 {
@@ -1091,6 +1103,7 @@ lnk_unwrap_rsp(Arena *arena, String8List arg_list)
       if (file_path_exists(name)) {
         // read rsp from disk
         String8 file = lnk_read_data_from_file_path(scratch.arena, 0, name);
+        file = lnk_text_file_string_from_data(scratch.arena, file);
         
         // parse rsp
         String8List rsp_args = lnk_arg_list_parse_windows_rules(scratch.arena, file);

src/linker/tests/linker_tests.c

@@ -2791,6 +2791,19 @@ TEST(import_export)
   //T_Ok(t_invoke_linkerf("/subsystem:console /entry:entry /out:a.exe /delayload:export.dll /export:entry kernel32.Lib delayimp.lib libcmt.lib export.lib import.obj entry.obj") == 0);
   // TODO: check import table
 }
+
+TEST(utf16_rsp)
+{
+  T_Ok(t_write_entry_obj());
+
+  String8  rsp_text = str8_lit("/subsystem:console /entry:entry /out:a.exe entry.obj\n");
+  String16 rsp16    = str16_from_8(arena, rsp_text);
+  String8  rsp_file = str8_cat(arena, str8_lit("\xff\xfe"), str8_array(rsp16.str, rsp16.size));
+  T_Ok(t_write_file(str8_lit("args.rsp"), rsp_file));
+
+  t_invoke_linkerf("@args.rsp");
+  T_Ok(g_last_exit_code == 0);
+}
 #endif
 
 TEST(image_base)