Ports: check for native python3 installation, add build script

For python3 cross compilation, a native installation of python3 is
needed. This patch adds a build script for python3 to the toolchain
and informs the user to run that script if the python port is build
and no native python3 with the same major and minor version is
being found.

Toolchain/BuildPython.sh

@@ -0,0 +1,62 @@
+#!/bin/bash
+set -e
+
+# This file will need to be run in bash, for now.
+
+DIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )"
+
+echo "$DIR"
+
+TARGET=i686-pc-serenity
+PREFIX="$DIR/Local"
+SYSROOT="$DIR/../Root"
+
+source "$DIR/../Ports/python-3.6/version.sh"
+
+echo PYTHON_VERSION is "$PYTHON_VERSION"
+echo PYTHON_URL is "$PYTHON_URL"
+
+echo PREFIX is "$PREFIX"
+echo SYSROOT is "$SYSROOT"
+
+mkdir -p "$DIR/Tarballs"
+
+source "$DIR/UseIt.sh"
+
+pushd "$DIR/Tarballs"
+    if [ ! -e "$PYTHON_ARCHIVE" ]; then
+        curl -O "$PYTHON_URL"
+    else
+        echo "Skipped downloading Python-$PYTHON_VERSION"
+    fi
+
+    md5="$(md5sum $PYTHON_ARCHIVE | cut -f1 -d' ')"
+    echo "python md5='$md5'"
+    if  [ "$md5" != "$PYTHON_MD5SUM" ] ; then
+        echo "python md5 sum mismatching, please run script again."
+        rm $PYTHON_ARCHIVE
+        exit 1
+    fi
+
+    if [ ! -d "Python-$PYTHON_VERSION" ]; then
+        echo "Extracting python..."
+        tar -xf "$PYTHON_ARCHIVE"
+    else
+        echo "Skipped extracting python"
+    fi
+popd
+
+mkdir -p "$PREFIX"
+mkdir -p "$DIR/Build/python"
+
+if [ -z "$MAKEJOBS" ]; then
+    MAKEJOBS=$(nproc)
+fi
+
+pushd "$DIR/Build/"
+    pushd python
+        "$DIR"/Tarballs/Python-$PYTHON_VERSION/configure --prefix="$PREFIX" || exit 1
+        make -j "$MAKEJOBS" || exit 1
+        make install || exit 1
+    popd
+popd