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