Latest nightly fails to build with feature simd

[viirya]

Describe the bug

After upgrading to latest nightly toolchain, cargo build failed.

To Reproduce
cargo +nightly build --all --features simd

error: expected one of `!` or `::`, found keyword `mod`
   --> /Users/liangchi/.cargo/registry/src/github.com-1ecc6299db9ec823/packed_simd_2-0.3.6/src/lib.rs:348:7
    |
348 | crate mod llvm {
    |       ^^^ expected one of `!` or `::`

error: could not compile `packed_simd_2` due to previous error
warning: build failed, waiting for other jobs to finish...

Expected behavior
The build command should work.

Additional context
@sunchao reported the issue at packed_simd: rust-lang/packed_simd#343.