Fix a build problem with power 8: use --with-system-zlib

Power 8 was once again having build issues. Adding --with-system-zlib
fixes them. It seems the builtin one is only needed when you are going
to build programs, and it falls apart in other cases.

Searching --with-system-zlib reveals this to be a very popular topic.

This has not broken other toolchain builds (for me); it should not for
anyone else. Then again, this is gcc, about which I need say no more.

Change-Id: Ica9d057d88982543b5dda471cc949c31fe15932f
Signed-off-by: Ronald G. Minnich <rminnich@gmail.com>
Reviewed-on: https://review.coreboot.org/13700
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
diff --git a/util/crossgcc/buildgcc b/util/crossgcc/buildgcc
index 3f41f98..a02056a 100755
--- a/util/crossgcc/buildgcc
+++ b/util/crossgcc/buildgcc
@@ -442,6 +442,7 @@
 		--enable-interwork --enable-multilib \
 		--disable-libatomic --disable-libcc1 --disable-decimal-float \
 		${GCC_OPTIONS} --enable-languages="${LANGUAGES}" \
+		--with-system-zlib \
 		--with-gmp=$DESTDIR$TARGETDIR --with-mpfr=$DESTDIR$TARGETDIR \
 		--with-mpc=$DESTDIR$TARGETDIR --with-libelf=$DESTDIR$TARGETDIR \
 		--with-pkgversion="coreboot toolchain v$CROSSGCC_VERSION $CROSSGCC_DATE" \