Currently we pick "weird" numbers like `1114112` for `None::<char>`. While that's not *wrong*, it's kinda *unnatural* -- a human wouldn't make that choice.
This PR instead picks `-1` for thinge like `None::<char>` -- like clang's `WEOF` -- and `None::<bool>` and such.
Any enums with more than one niched value (so not `Result` nor `Option`) remain as they were before.
Resolve several ./x test failures on RISC-V caused by ABI and codegen
differences. Update multiple codegen-llvm tests for compatibility, and
explicitly set the target for transmute-scalar.rs to x86_64 to ensure
consistent behavior across hosts.
Scott McMurray