Expand use and support for LTO and FDO builds

Adds LTO to the CI build for Linux, which bring it as close as possible
to the planned formal release, which will additionally use FDO.

Adds some helper scripts to work with FDO files.

Improves the build notes for how to create and use FDO files.

.github/workflows/linux.yml

@@ -96,11 +96,17 @@ jobs:
           echo ::set-env name=VERSION::$VERSION
       - name: Build
         env:
-          FLAGS: -O3 -DNDEBUG -pipe
+          AR: gcc-ar
+          CC: ccache gcc
+          CXX: ccache g++
+          LD: gcc
+          RANLIB: gcc-ranlib
+          FLAGS: -O3 -flto -ffunction-sections -fdata-sections -DNDEBUG -pipe
+          LINKFLAGS: -Wl,--as-needed
         run: |
           set -x
           ./autogen.sh
-          ./configure CC="ccache gcc" CXX="ccache g++" CFLAGS="$FLAGS" CXXFLAGS="$FLAGS" --disable-screenshots
+          ./configure CFLAGS="$FLAGS" CXXFLAGS="$FLAGS" LDFLAGS="$FLAGS $LINKFLAGS -flto=$(nproc)" --disable-screenshots
           make -j "$(nproc)"
           strip src/dosbox
       - name: Package