crossgcc: Support OSX 10.9 built-in tar utility program.

Unlike OSX 10.8, OSX 10.9 doesn't provide GNU tar program, and built-in
tar program is bsdtar 2.8.3. bsdtar can build crossgcc toolchain.
Modify buildgcc to support tar in OSX 10.9 (uname = Darwin).

Change-Id: I093898f8f99e29918387f9b275a30af461a7e1be
Signed-off-by: Andrew Wu <arw@dmp.com.tw>
Reviewed-on: http://review.coreboot.org/5598
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Patrick Georgi <patrick@georgi-clan.de>
diff --git a/util/crossgcc/buildgcc b/util/crossgcc/buildgcc
index c6d7183..5aa98d5 100755
--- a/util/crossgcc/buildgcc
+++ b/util/crossgcc/buildgcc
@@ -105,8 +105,9 @@
 			fi
 		fi
 	done
-	# A workaround for XxxBSD, whose nongnu patch and tar also work.
-	if [ $UNAME = "FreeBSD" -o $UNAME = "NetBSD" ]; then
+	# A workaround for OSX 10.9 and some BSDs, whose nongnu
+	# patch and tar also work.
+	if [ $UNAME = "Darwin" -o $UNAME = "FreeBSD" -o $UNAME = "NetBSD" ]; then
 		if [ $1 != "make" ]; then
 			if test -x "`which $1 2>/dev/null`"; then
 				echo $1