All supported platforms should be built and tested in CI

We presently declare the following support:

Target triple |Binary|Library|
------------------------------|:----:|------|
x86_64-unknown-linux-gnu | ✓ | ✓ |
aarch64-unknown-linux-gnu | ✓ | ✓ |
armv7-unknown-linux-gnueabihf | ✓ | ✓ |
x86_64-apple-darwin | ✓ | ✓ |
x86_64-pc-windows-msvc | | ✓ |
aarch64-apple-ios | | ✓ |
armv7-apple-ios | | ✓ |
armv7s-apple-ios | | ✓ |
aarch64-linux-android | | ✓ |
arm-linux-androideabi | | ✓ |

However, we only build and test on, afaict, `x86_64-unknown-linux-gnu`, `x86_64-pc-windows-msvc`, and `aarch64-apple-darwin` (which is notably not declared as supported!).

Unsurprisingly, the targets that don't get tested have rotted:
- #428 - android builds reportedly broken
- #450 and #460 - platforms lacking native 64 bit atomics have been broken for years
- #383 - notes lack of declared support for aarch64-apple-darwin (even though it's what we build in CI 🙃

Broadly:
- we should be building and testing at least one 32 bit target
- we should be building and testing android, if possible
- the support list should be pruned to things we can routinely test