v1.1.3: portable-atomic polyfill so mipsel-softfloat compiles (#46)

v1.1.2 reached cargo build inside the mipsel docker this time
(YAML-fold bug finally out of the way) and surfaced the real
underlying problem: MIPS32 has no native 64-bit atomic instructions,
so std::sync::atomic::AtomicU64 doesn't exist on
mipsel-unknown-linux-musl. Three call sites (DomainFronter stats
counters + the request-cache) failed to resolve the import.

Fix: depend on `portable-atomic` with the `fallback` feature and
import AtomicU64 from there instead of std. The API is identical
(same associated methods, same Ordering accepted), so the two
touched files change only the `use` line. On 64-bit targets
portable-atomic compiles down to the native 64-bit atomic insns
with no overhead; on MIPS32 it uses a global spinlock, which is
fine for counter increments that happen a few times per relay.

Cache.rs and domain_fronter.rs both updated. No other callers of
AtomicU64 in non-cfg-gated code (android_jni.rs has it but is
gated `#![cfg(target_os = "android")]`, so mipsel-linux-musl
never sees it).

`cargo test --lib` / `cargo build` still pass on host.

Cargo.lock

@@ -2199,6 +2199,7 @@ dependencies = [
  "httparse",
  "jni 0.21.1",
  "libc",
+ "portable-atomic",
  "rand 0.8.6",
  "rcgen",
  "rustls",