Add a VISIBILITY_HIDDEN option, which can be used to make all compiler-rt function definitions hidden/private extern.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@93790 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/make/lib_info.mk b/make/lib_info.mk
index 91ee23b..a54152a 100644
--- a/make/lib_info.mk
+++ b/make/lib_info.mk
@@ -46,4 +46,5 @@
 # The names of all the available options.
 AvailableOptions := AR ARFLAGS \
                     CC CFLAGS FUNCTIONS OPTIMIZED \
-                    RANLIB RANLIBFLAGS
+                    RANLIB RANLIBFLAGS \
+                    VISIBILITY_HIDDEN