Add libc constraint and libc aware toolchains

- Also get rid of @bazel_skylib dependency

toolchain/private/defs.bzl

@@ -28,6 +28,8 @@ _GLIBCS = [
     "2.34",
 ]
 
+LIBCS = ["musl", "gnu"] + ["gnu.{}".format(glibc) for glibc in _GLIBCS]
+
 def target_structs():
     ret = []
     for zigcpu, gocpu in (("x86_64", "amd64"), ("aarch64", "arm64")):
@@ -100,6 +102,7 @@ def _target_linux_gnu(gocpu, zigcpu, glibc_version = ""):
             "@platforms//os:linux",
             "@platforms//cpu:{}".format(zigcpu),
         ],
+        libc_constraint = "@zig_sdk//libc:{}".format(glibc_suffix),
         tool_paths = {"ld": "ld.lld"},
     )
 
@@ -120,5 +123,6 @@ def _target_linux_musl(gocpu, zigcpu):
             "@platforms//os:linux",
             "@platforms//cpu:{}".format(zigcpu),
         ],
+        libc_constraint = "@zig_sdk//libc:musl",
         tool_paths = {"ld": "ld.lld"},
     )