fix a build break on x86 targets without guaranteed SSE2 support

This is quite hard to trigger, because SSE2 has been guaranteed for a
long time. But you could trigger it this way:

    rustup target add i686-unknown-linux-musl
    RUSTFLAGS="-C target-cpu=i386" cargo build --target i686-unknown-linux-musl

Note a relevant gotcha though: The `cross` tool will not forward
environment variables like RUSTFLAGS to the container by default, so if
you're testing with `cross` you'll need to use the `rustc` command to
explicitly pass the flag, as I've done here in ci.yml. (Or you could
create a `Cross.toml` file, but I don't want to commit one of those if I
can avoid it.)

1 files changed, 2 insertions, 0 deletions

diff --git a/src/platform.rs b/src/platform.rs
index c0eb21d..ee9c349 100644
--- a/src/platform.rs
+++ b/src/platform.rs
@@ -403,6 +403,8 @@ pub fn sse2_detected() -> bool {
     {
         return true;
     }
+    #[allow(unreachable_code)]
+    false
 }
 
 #[inline(always)]