Toolchain: Introduce stubs for core system libraries

This allows the linker to link against these dynamic libraries when
compiling libc++/libunwind, without having to do a separate
bootstrapping LibC build.

Without this change, libc++ would fail to pick up the need to link to
`LibPthread` if no prior builds of it existed. Because of this, we'd
immediately have an assertion failure in SystemServer, as mutexes are
used for the safe construction of function-local static variables.

Toolchain/Stubs/README.md

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+# Library stubs
+
+This directory contains stubs for SerenityOS libraries (LibC, LibM, LibDl, LibPthread)
+that are referenced from the LLVM runtime libraries. These are needed by the linker
+in order to add the required `DT_NEEDED` entries.
+
+## Do these need to be updated?
+
+Generally, no. LLVM does not use the header files to decide which functionality it can
+use. After adding a new function to a header, you don't have to worry about LLVM
+toolchain builds failing because the symbol is not present in the stubs.
+
+## How to generate these?
+
+First, you need to have a working SerenityOS installation that's been built by the
+Clang toolchain. Then, using the `llvm-ifs` tool, these libraries need to be converted
+into a stripped-down stub form. To do that, run the following command:
+
+```sh
+Toolchain/Local/clang/bin/llvm-ifs --output-format=ELF --output=<path-to-stub> <path-to-original>
+```