std.Build: avoid use of catch unreachable

Usage of `catch unreachable` in build scripts is completely harmless
because build scripts are always run in Debug mode, however, it sets a
poor example for beginners to learn from.

lib/std/Build/CheckFileStep.zig

@@ -18,7 +18,7 @@ pub fn create(
     source: std.Build.FileSource,
     expected_matches: []const []const u8,
 ) *CheckFileStep {
-    const self = builder.allocator.create(CheckFileStep) catch unreachable;
+    const self = builder.allocator.create(CheckFileStep) catch @panic("OOM");
     self.* = CheckFileStep{
         .builder = builder,
         .step = Step.init(.check_file, "CheckFile", builder.allocator, make),