gollvm: tweak .gitignore

Tweak the ignore rules for libffi and libbacktrace to allow for
the possibility that they are symbolic links as opposed to
actual git repos.

Change-Id: Id9e9f7debca2f5a16d6e1f4cc0456de921f8df25
Reviewed-on: https://go-review.googlesource.com/c/159137
Reviewed-by: Cherry Zhang <cherryyz@google.com>
diff --git a/.gitignore b/.gitignore
index 0f85da1..43c3309 100644
--- a/.gitignore
+++ b/.gitignore
@@ -4,6 +4,8 @@
 
 # These are separate git repos and are tracked independently
 gofrontend/
+libgo/libbacktrace
+libgo/libffi
 libgo/libbacktrace/
 libgo/libffi/