gn build: Build compiler-rt code with -fvisibility=hidden.

This matches the CMake build.

Differential Revision: https://reviews.llvm.org/D69202

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@375299 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/utils/gn/build/BUILD.gn b/utils/gn/build/BUILD.gn
index 7ad429a..b06ef9a 100644
--- a/utils/gn/build/BUILD.gn
+++ b/utils/gn/build/BUILD.gn
@@ -221,6 +221,7 @@
     "-fPIC",
     "-funwind-tables",
     "-gline-tables-only",
+    "-fvisibility=hidden",
   ]
 }