Support a custom libc configuration file

[aherrmann]

See https://github.com/aherrmann/rules_zig/issues/161#issuecomment-1872501510.

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.