feat: Enable libc/libc++ linking to expose system headers

[aherrmann]

The Zig compiler ships with various versions of libc and libc++. However, it does not link against these libraries by default, and accordingly does not expose standard library headers for C imports by default. To enable the import of C standard library headers the compiler must be instructed to link against libc or libc++ as appropriate by passing the -lc or -lc++ options, or similar.

This is already possible with rules_zig as is by adding a cc_library target to the cdeps attribute that sets linkopts to include -lc or similar. This change adds helper targets to rules_zig for convenience and includes an e2e test for this use-case.

Closes #161

• test: C standard library header include
• Helper targets to enable libc/libc++ linking
• Use helper targets in tests
• Mention libc/libc++ helper targets in doc strings
• Update generated files