Esp32 support

[tombrk]

This library already works really well on the embedded ESP32 SOC, but that platform being 32-bit means we cannot use AtomicU32.

This PR adds xtensa to the list of 32-bit architectures, so that it compiles.
It also implements Atomic for f32, so that Gauges become usable.

IMHO we really should use rust-lang/rust#32976 once available, but that is for another day.

[mxinden]

Thanks for providing a patch!

Would you mind adding a changelog entry? Something along the lines of:

## [0.15.2] - unreleased

### Added

- Support for `f32` `Gauge` and `Counter`. See [PR xx].
- Support for `xtensa` architecture. See [PR xx].

And bump the patch version in Cargo.toml?

Also does Rust support cross compilation for xtensa? If so, want to add it to the matrix in our CI config?