Toolchain: Install system headers before building GCC

GCC determines whether the system's <limits.h> header is usable
and installs a different version of its own <limits.h> header
depending on whether the system header file exists.

If the system header is missing GCC's <limits.h> header does not
include the system header via #include_next.

For this to work we need to install LibC's headers before
attempting to build GCC.

Also, re-running BuildIt.sh "hides" this problem because at that
point the sysroot directory also already has a <limits.h> header
file from the previous build.

Toolchain/BuildIt.sh

@@ -239,15 +239,6 @@ pushd "$DIR/Build/$ARCH"
  --enable-lto \
  ${TRY_USE_LOCAL_TOOLCHAIN:+"--quiet"} || exit 1
 
- echo "XXX build gcc and libgcc"
- "$MAKE" -j "$MAKEJOBS" all-gcc || exit 1
- if [ "$(uname -s)" = "OpenBSD" ]; then
- ln -sf liblto_plugin.so.0.0 gcc/liblto_plugin.so
- fi
- "$MAKE" -j "$MAKEJOBS" all-target-libgcc || exit 1
- echo "XXX install gcc and libgcc"
- "$MAKE" install-gcc install-target-libgcc || exit 1
-
  echo "XXX serenity libc and libm headers"
  mkdir -p "$BUILD"
  pushd "$BUILD"
@@ -261,6 +252,15 @@ pushd "$DIR/Build/$ARCH"
  unset SRC_ROOT
  popd
 
+ echo "XXX build gcc and libgcc"
+ "$MAKE" -j "$MAKEJOBS" all-gcc || exit 1
+ if [ "$(uname -s)" = "OpenBSD" ]; then
+ ln -sf liblto_plugin.so.0.0 gcc/liblto_plugin.so
+ fi
+ "$MAKE" -j "$MAKEJOBS" all-target-libgcc || exit 1
+ echo "XXX install gcc and libgcc"
+ "$MAKE" install-gcc install-target-libgcc || exit 1
+
  echo "XXX build libstdc++"
  "$MAKE" -j "$MAKEJOBS" all-target-libstdc++-v3 || exit 1
  echo "XXX install libstdc++"