Fix syscall definitions for x86_64-unknown-linux-gnux32.

[hvdijk]

The x32 ABI uses the 0x40000000 bit in syscall numbers. Additionally, the x32 ABI keeps syscall numbers as type long, but its type long is i32 rather than i64.

[lucab]

Thanks for the PR, happy to merge this.
However it would be helpful to have in the commit message some URL references to kernel code for those x32 details, so that it's easy to review and follow through history in the future.

[hvdijk]

Sure, more elaborate descriptions are good. I think, but may be mistaken, that the syscall taking i32 may be a userland decision more than a kernel decision, so have included references to rust-lang/libc instead, I hope that is okay too.

I noticed rust-lang/libc already exposes SYS_capget and SYS_capset, so an alternative fix could be to just use those directly.

[lucab]

I noticed rust-lang/libc already exposes SYS_capget and SYS_capset, so an alternative fix could be to just use those directly.

Yes, it is probably a good idea to do that too anyway. I don't remember why I didn't go that route initially, perhaps libc was not exposing those constants when I started this crate (or I plainly missed that).

[nabijaczleweli]

This PR is the only one standing before getting x32 support into libc now, would you mind publishing a new version, please?