trees: only symlink host gcc/gnat to build xgcc

In general, we don't want to mess with the hostcc, unless
we have to. To avoid other breakage, clear what we did
after crossgcc has compiled.

This is a follow-up to the previous patches, matching gcc
to gnat versions and vice versa, when compiling crossgcc.

Signed-off-by: Leah Rowe <leah@libreboot.org>

script/trees

@@ -216,6 +216,9 @@ check_cross_compiler()
 		make -C "$cbdir" crossgcc-$xfix $xgccargs || \
 		    make -C "$cbdir" crossgcc-$xfix $xgccargs || \
 		    $err "!mkxgcc $project/$xtree '$xfix' '$xgccargs'"
+
+		# we only want to mess with hostcc to build xgcc
+		rm -f xbmkpath/* || $err "Cannot clear xbmkpath/"; :
 	done; return 0
 }