Toolchain: Add support for building the userland with the mold linker

This commit adds support for building the SerenityOS userland with the
new [mold linker].

This is not enabled by default yet; to link using mold, run the
`Toolchain/BuildMold.sh` script to build the latest release of mold, and
set the `ENABLE_MOLD_LINKER` CMake variable to ON. This option relies on
toolchain support that has been added just recently, so you might need
to rebuild your toolchain for mold to work.

[mold linker]: https://github.com/rui314/mold

Toolchain/BuildMold.sh

@@ -0,0 +1,44 @@
+#!/usr/bin/env bash
+
+# This script builds the mold linker that can optionally be used for linking
+# the SerenityOS userland.
+set -e
+
+DIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )"
+
+NPROC="nproc"
+SYSTEM_NAME="$(uname -s)"
+
+if [ "$SYSTEM_NAME" = "OpenBSD" ]; then
+    NPROC="sysctl -n hw.ncpuonline"
+elif [ "$SYSTEM_NAME" = "FreeBSD" ]; then
+    NPROC="sysctl -n hw.ncpu"
+elif [ "$SYSTEM_NAME" = "Darwin" ]; then
+    NPROC="sysctl -n hw.ncpu"
+fi
+
+[ -z "$MAKEJOBS" ] && MAKEJOBS=$($NPROC)
+
+mkdir -p "$DIR"/Tarballs
+pushd "$DIR"/Tarballs
+
+if [ "$1" = "--git" ]; then
+    [ ! -d mold ] && git clone https://github.com/rui314/mold.git
+
+    cd mold
+
+    git pull
+else
+    VERSION=1.0.2
+    [ ! -e mold-$VERSION.tar.gz ] && curl -L "https://github.com/rui314/mold/archive/refs/tags/v$VERSION.tar.gz" -o mold-$VERSION.tar.gz
+    [ ! -e mold-$VERSION ] && tar -xzf mold-$VERSION.tar.gz
+    cd mold-$VERSION
+fi
+
+make clean
+export DESTDIR="$DIR"/Local/mold
+export PREFIX=
+make -j "$MAKEJOBS"
+make install
+
+popd