Support a custom libc configuration file

[aherrmann]

See #161 (comment).

Zig links against its own libc. However, this is configurable. You can either select one of the builtin libc's by adjusting the ABI part of the target triple in a custom target toolchain. Or, in principle Zig can also accept a custom libc configuration, which could also be exposed through a target toolchain attribute.

  --libc [file]             Provide a file which specifies libc paths

Add an optional libc attribute to the target toolchain rule. When set the --libc flag will be passed to the Zig compiler accordingly.