Deprecate 32-bit targets

[swiftcoder]

As of iOS 11, 32-bit apps are no longer supported on iOS. Removing the 32-bit targets would save considerable compilation time, and also allow programs too large for 32-bit to be lipo'd without running into this sort of error:

LLVM ERROR: Section too large, can't encode r_address (0x1005de7) into 24 bits of scattered relocation entry.

[swiftcoder]

For the record, I do know that one can do this manually by specifying just the 64-bit targets on the command line:

cargo lipo --targets=aarch64-apple-ios,x86_64-apple-ios --release

But going forward it'd be nice to have the default behaviour be the forward-compatible one.