Remove or revamp sys::syscall module

[kamalmarhubi]

This came up in a comment from @pnkfelix on #339.

This module appears to be Linux specific. It declares

extern {
    pub fn syscall(num: Syscall, ...) -> c_int;
}

for all architectures, with Syscall being architecture dependent alias that is c_long on all listed architectures.

On Linux, [syscall(2)][syscall] should return a long, so this declaration is incorrect. We should switch to using definitions from libc and tidy up or remove this module.

Related: #264

[kamalmarhubi]

This PR is related: rust-lang/libc#374

[Susurrus]

Closed by #747