Add support for direct system calls on AMD64 Linux

This has the unfortunate effect that all projects using rust-core
need #[feature(asm)]; at the crate root.  I thought about putting
syscalls behind a --cfg flag for this reason.  However I expect
we'll use asm!() for other things too, and it's an implementation
detail which doesn't really relate to the set of rust-core features
in use.  So I think for now we can just accept #[feature(asm)];
until we can build rust-core as a separate library crate.

Some code adapted from https://github.com/Aatch/grease-bench
with permission (conversation with aatch in #rust, 2013-10-22)
Add macros.rs with a system call macro
Add an example Linux no-libc program
Add gitignore