Add vlibc scaffold, build system, and documentation

Establish the repository layout per Vox guidelines (layouts/C.md,
building/C.md): Autotools build (GCC-only, C23), five compatibility
profiles (--enable-onlyposix/--enable-muslmimic/--enable-muslext/
--enable-spoof, default vlibc), alongside/overwrite install methods,
vlibc-gcc/vlibc-clang drivers, static-linking requirement (except
spoof), benchmark harness, and tooling (.clang-format/.clang-tidy/
.clangd + compile_commands.json).

tools/vlibc-gcc.in

@@ -0,0 +1,24 @@
+#!/bin/sh
+# vlibc-gcc — compile and link against vlibc using GCC.
+#
+# Generated by configure from tools/vlibc-gcc.in. Do not edit directly.
+# This is the compiler driver that vlibc ships for its consumers.
+
+prefix='@prefix@'
+install_mode='@vlibc_install_mode@'
+profile='@vlibc_profile@'
+
+case "$install_mode" in
+    overwrite)
+        vlibc_includedir="$prefix/include"
+        vlibc_libdir="$prefix/lib"
+        ;;
+    *)
+        vlibc_includedir="$prefix/lib/vlibc/include"
+        vlibc_libdir="$prefix/lib/vlibc/lib"
+        ;;
+esac
+
+# TODO: full sysroot handling for `overwrite` installs and cross-compilation,
+# and -static/-lvlibc wiring once the library exports its full ABI.
+exec gcc -isystem "$vlibc_includedir" -L"$vlibc_libdir" "$@"