Bake in default sysroots appropriately

[pcwalton]

We should bake in a default sysroot into the binary where rustc looks for all of its stuff (stdlib, glue, etc).

[espindola]

The driver currently looks at paths relative to itself. Is this what you have in mind?

[brson]

The driver now finds the absolute path to itself and considers the directory one level higher the sysroot. So the sysroot of /usr/bin/rustc is /usr/bin.