Conditionally compile gix-revision at_symbol fuzzed test

This allows compilation to succeed when building tests on 32-bit
Windows systems, and probably other 32-bit systems/builds.

This test function had been set as ignored in 32-bit builds with
the `cfg_attr` attribute. But it was still compiled, and the
compilation failed on systems where the literal 9223372036854775808
is too big for `usize`.

This was due to the implicit `#[deny(overflowing_literals)]`. While
that could be conditionally suppressed for 32-bit builds, the test
would not really be meaningful. So this leaves that diagnostic in
place, and turns the conditional ignore into conditional
compilation.

Author: EliahKagan

gix-revision/tests/spec/parse/anchor/at_symbol.rs

@@ -11,7 +11,7 @@ fn braces_must_be_closed() {
 }
 
 #[test]
-#[cfg_attr(target_pointer_width = "32", ignore = "Only works this way on 64 bit systems")]
+#[cfg(target_pointer_width = "64")] // Only works this way on 64-bit systems.
 fn fuzzed() {
     let rec = parse("@{-9223372036854775808}");
     assert_eq!(rec.nth_checked_out_branch, [Some(9223372036854775808), None]);