gollvm: update libgo cmake rules for gofrontend CL 454176

Update libgo cmake rules to adapt to recent gofrontend CL 454176,
which introduces some new makefile changes for strerror.

Change-Id: I400540b47806e9e39f41f531ad897607b3b1de81
Reviewed-on: https://go-review.googlesource.com/c/gollvm/+/456016
Reviewed-by: Ian Lance Taylor <iant@google.com>
diff --git a/libgo/CMakeLists.txt b/libgo/CMakeLists.txt
index 8625699..6fcdff6 100644
--- a/libgo/CMakeLists.txt
+++ b/libgo/CMakeLists.txt
@@ -482,6 +482,7 @@
   "runtime/go-reflect-call.c"
   "runtime/go-setenv.c"
   "runtime/go-signal.c"
+  "runtime/go-strerror.c"
   "runtime/go-unsafe-pointer.c"
   "runtime/go-unsetenv.c"
   "runtime/go-unwind.c"