allow(unreachable_code) in all the *_detected functions

Previously we only disabled these warnings for SSE2, which is assumed
enabled on x86-64, but it looks like new nightly compilers are also
assuming SSE4.1 on macOS. Disabling these warnings across all the
detection functions accounts for that, and it also gets rid of some
warnings that you'd see if you used RUSTFLAGS='-C target-cpu=native'.

src/platform.rs

@@ -327,6 +327,7 @@ impl Platform {
 #[cfg(blake3_avx512_ffi)]
 #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
 #[inline(always)]
+#[allow(unreachable_code)]
 pub fn avx512_detected() -> bool {
     // A testing-only short-circuit.
     if cfg!(feature = "no_avx512") {
@@ -349,6 +350,7 @@ pub fn avx512_detected() -> bool {
 
 #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
 #[inline(always)]
+#[allow(unreachable_code)]
 pub fn avx2_detected() -> bool {
     // A testing-only short-circuit.
     if cfg!(feature = "no_avx2") {
@@ -371,6 +373,7 @@ pub fn avx2_detected() -> bool {
 
 #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
 #[inline(always)]
+#[allow(unreachable_code)]
 pub fn sse41_detected() -> bool {
     // A testing-only short-circuit.
     if cfg!(feature = "no_sse41") {