Add libc constraint and libc aware toolchains

- Also get rid of @bazel_skylib dependency

test/c/BUILD

@@ -0,0 +1,46 @@
+load("@bazel-zig-cc//rules:platform.bzl", "platform_binary")
+
+cc_binary(
+    name = "which_libc",
+    srcs = ["main.c"],
+    target_compatible_with = [
+        "@platforms//os:linux",
+    ],
+)
+
+[
+    (
+        platform_binary(
+            name = "which_libc_{}".format(name),
+            src = "which_libc",
+            extra_toolchains = [
+                # toolchains for specific libc versions
+                "//libc_aware/toolchain:linux_amd64_gnu.2.19",
+                "//libc_aware/toolchain:linux_amd64_gnu.2.28",
+                "//libc_aware/toolchain:linux_amd64_gnu.2.31",
+                "//libc_aware/toolchain:linux_amd64_musl",
+                # fallback toolchains
+                "//toolchain:linux_amd64_gnu.2.19",
+                "//toolchain:linux_arm64_gnu.2.28",
+            ],
+            platform = platform,
+        ),
+        sh_test(
+            name = "test_libc_{}".format(name),
+            srcs = ["test.sh"],
+            data = ["which_libc_{}".format(name)],
+            env = {
+                "WANT": want,
+                "BINARY": "$(location which_libc_{})".format(name),
+            },
+        ),
+    )
+    for name, platform, want in [
+        ("linux_amd64_musl", "//libc_aware/platform:linux_amd64_musl", "non-glibc"),
+        ("linux_amd64_gnu.2.19", "//libc_aware/platform:linux_amd64_gnu.2.19", "glibc_2.19"),
+        ("linux_amd64_gnu.2.28", "//libc_aware/platform:linux_amd64_gnu.2.28", "glibc_2.28"),
+        ("linux_amd64_gnu.2.31", "//libc_aware/platform:linux_amd64_gnu.2.31", "glibc_2.31"),
+        ("linux_amd64", "//platform:linux_amd64", "glibc_2.19"),
+        ("linux_arm64", "//platform:linux_arm64", "glibc_2.28"),
+    ]
+]

test/c/main.c

@@ -0,0 +1,11 @@
+#include <stdio.h>
+#include <features.h>
+
+int main() {
+    #ifdef __GLIBC__
+    printf("glibc_%d.%d", __GLIBC__, __GLIBC_MINOR__);
+    #else
+    puts("non-glibc");
+    #endif
+    return 0;
+}

test/c/test.sh

@@ -0,0 +1,16 @@
+#/bin/bash
+
+set -euo pipefail
+
+want=$WANT
+binary=$BINARY
+
+got=$($binary)
+
+if [[ "$got" != "$want" ]]; then
+    echo wanted:
+    echo \ \ "$want"
+    echo got:
+    echo \ \ "$got"
+    exit 1
+fi