Add singlethreaded fence intrinsics.

[tari]

These new intrinsics are comparable to atomic_signal_fence in C++,
ensuring the compiler will not reorder memory accesses across the
barrier, nor will it emit any machine instructions for it.

[rust-highfive]

r? @alexcrichton

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[alexcrichton]

Currently we are planning to remove the entire intrinsics module from the public-facing API of the standard library, in which case we may not want to add these at this time. I think that when adding these intrinsics they would also want to be exposed in a public-facing soon-to-be-stable location as well, but this sort of addition to the standard library requires an RFC.

In short, I believe that an RFC is needed for these changes. Feel free to open an issue in the RFC repo, and I could also help out with writing an RFC if you need it!