Web: Refactor build script to run in parallel with full LTO

Fabio found that full LTO produces smaller binaries than ThinLTO,
and that's quite important for the Web platform.

But for an obscure reason LLVM's full LTO cannot linking with multiple
threads, and so it's slow as heck (10-15 min per build).

So we work it around by starting all builds in parallel so that they
can all link at the same time.

The code to do so is pretty ugly, could be refactored further.

build-web/build.sh

@@ -4,9 +4,20 @@ set -e
 
 # Config
 
-export SCONS="scons -j${NUM_CORES} verbose=yes warnings=no progress=no"
+# To speed up builds with single-threaded full LTO linking,
+# we run all builds in parallel each from their own folder.
+export NUM_JOBS=5
+declare -a JOBS=(
+  "tools=yes target=release_debug use_closure_compiler=yes"
+  "tools=no target=release_debug"
+  "tools=no target=release"
+  "tools=no target=release_debug dlink_enabled=yes"
+  "tools=no target=release dlink_enabled=yes"
+)
+
+export SCONS="scons -j$(expr ${NUM_CORES} / ${NUM_JOBS}) verbose=yes warnings=no progress=no"
 export OPTIONS="production=yes"
-export OPTIONS_MONO="module_mono_enabled=yes use_lto=no"
+export OPTIONS_MONO="module_mono_enabled=yes -j${NUM_CORES}"
 export TERM=xterm
 
 source /root/emsdk/emsdk_env.sh
@@ -21,21 +32,25 @@ tar xf /root/godot.tar.gz --strip-components=1
 if [ "${CLASSICAL}" == "1" ]; then
   echo "Starting classical build for Web..."
 
-  $SCONS platform=web ${OPTIONS} target=release_debug tools=no
-  $SCONS platform=web ${OPTIONS} target=release tools=no
+  for i in {0..4}; do
+    cp -r /root/godot /root/godot$i
+    cd /root/godot$i
+    echo "$SCONS platform=web ${OPTIONS} ${JOBS[$i]}"
+    $SCONS platform=web ${OPTIONS} ${JOBS[$i]} &
+    pids[$i]=$!
+  done
 
-  $SCONS platform=web ${OPTIONS} target=release_debug tools=no dlink_enabled=yes
-  $SCONS platform=web ${OPTIONS} target=release tools=no dlink_enabled=yes
-
-  mkdir -p /root/out/templates
-  cp -rvp bin/*.zip /root/out/templates
-  rm -f bin/*.zip
-
-  $SCONS platform=web ${OPTIONS} target=release_debug tools=yes use_closure_compiler=yes
+  for pid in ${pids[*]}; do
+    wait $pid
+  done
 
   mkdir -p /root/out/tools
-  cp -rvp bin/*.zip /root/out/tools
-  rm -f bin/*.zip
+  cp -rvp /root/godot0/bin/*tools*.zip /root/out/tools
+
+  mkdir -p /root/out/templates
+  for i in {1..4}; do
+    cp -rvp /root/godot$i/bin/*.zip /root/out/templates
+  done
 fi
 
 # Mono