mark arm intrinsics as safe

[usamoi]

This patch marks all neon intrinsics that do not operate on memory, crc and crypto intrinsics as safe.

ACP: rust-lang/libs-team#494

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[Amanieu]

ARM maintainers should have a second pass at reviewing this.

cc @JamieCunliffe @adamgemmell @Jamesbarford

[Jamesbarford]

It might be beneficial to have #1708 merged prior to these changes as #1708 converts most of the manually defined intrinsics to the YAML file.

[Amanieu]

This needs another rebase since the f16 intrinsics were added.

[Jamesbarford]

LGTM