prepare: disable preserve-libs for glibc

Since installing _sys-libs/glibc_ in the crossdev root overwrites the
libraries built by crossdev, Portage records the latter as needing to be
protected.  This results in _everything_ being pulled in to
@preserved-rebuild, which ultimately does nothing since the preserved
library is never replaced.  To avoid this pointlessness, we need to
disable the _preserve-libs_ feature when reinstalling _glibc_.

We also disable _protect-owned_ to avoid spam from Portage when
initially overwriting the libraries and headers in the crossdev root.

prepare.sh

@@ -42,7 +42,7 @@ if [ ! -f portage/make.conf/10-crossdev.conf ]; then
         > "${O}"/portage/etc/portage/make.conf/10-crossdev.conf
 fi
 
-FEATURES="${FEATURES}" \
+FEATURES="${FEATURES} -preserve-libs -protect-owned" \
 PKGDIR="${PKGDIR}" \
 ${target}-emerge -bk1nvj sys-libs/glibc