Provide a detailed message for invalid arch in target triple (#21921)

lib/std/Target/Query.zig

@@ -193,6 +193,9 @@ pub const ParseOptions = struct {
 
         /// If error.UnknownCpuFeature is returned, this will be populated.
         unknown_feature_name: ?[]const u8 = null,
+
+        /// If error.UnknownArchitecture is returned, this will be populated.
+        unknown_architecture_name: ?[]const u8 = null,
     };
 };
 
@@ -208,8 +211,10 @@ pub fn parse(args: ParseOptions) !Query {
     const arch_name = it.first();
     const arch_is_native = mem.eql(u8, arch_name, "native");
     if (!arch_is_native) {
-        result.cpu_arch = std.meta.stringToEnum(Target.Cpu.Arch, arch_name) orelse
+        result.cpu_arch = std.meta.stringToEnum(Target.Cpu.Arch, arch_name) orelse {
+            diags.unknown_architecture_name = arch_name;
             return error.UnknownArchitecture;
+        };
     }
     const arch = result.cpu_arch orelse builtin.cpu.arch;
     diags.arch = arch;

lib/std/zig.zig

@@ -660,6 +660,17 @@ pub fn parseTargetQueryOrReportFatalError(
             }
             fatal("unknown object format: '{s}'", .{opts.object_format.?});
         },
+        error.UnknownArchitecture => {
+            help: {
+                var help_text = std.ArrayList(u8).init(allocator);
+                defer help_text.deinit();
+                inline for (@typeInfo(std.Target.Cpu.Arch).@"enum".fields) |field| {
+                    help_text.writer().print(" {s}\n", .{field.name}) catch break :help;
+                }
+                std.log.info("available architectures:\n{s} native\n", .{help_text.items});
+            }
+            fatal("unknown architecture: '{s}'", .{diags.unknown_architecture_name.?});
+        },
         else => |e| fatal("unable to parse target query '{s}': {s}", .{
             opts.arch_os_abi, @errorName(e),
         }),