Upgrade packed_simd to 3.6.7 for arrow2

[BohuTANG]

packed_simd v3.6.7 fixed the llvm_asm issue:
rust-lang/packed_simd@6d3c23e

After the upgrade, we can upgrade rust toolchain now.

[BohuTANG]

cc @jorgecarleitao

[jorgecarleitao]

Thanks! arrow2 depends on 0.3, so the patch bump should work out of the box.

Note that I am fixing portable-simd here rust-lang/portable-simd#238 to allow arrow2 to depend on it (see jorgecarleitao/arrow2#747 and last comment there)