[scudo] Add default implementations for weak functions

Summary:
This is in preparation for platforms where `SANITIZER_SUPPORTS_WEAK_HOOKS` is 0.
They require a default implementation.

Reviewers: alekseyshl

Reviewed By: alekseyshl

Subscribers: delcypher, llvm-commits, #sanitizers

Differential Revision: https://reviews.llvm.org/D42557

llvm-svn: 323795
diff --git a/compiler-rt/lib/scudo/scudo_flags.cpp b/compiler-rt/lib/scudo/scudo_flags.cpp
index 2aff3ef..ad50800 100644
--- a/compiler-rt/lib/scudo/scudo_flags.cpp
+++ b/compiler-rt/lib/scudo/scudo_flags.cpp
@@ -12,13 +12,12 @@
 //===----------------------------------------------------------------------===//
 
 #include "scudo_flags.h"
+#include "scudo_interface_internal.h"
 #include "scudo_utils.h"
 
 #include "sanitizer_common/sanitizer_flags.h"
 #include "sanitizer_common/sanitizer_flag_parser.h"
 
-SANITIZER_INTERFACE_WEAK_DEF(const char*, __scudo_default_options, void);
-
 namespace __scudo {
 
 static Flags ScudoFlags;  // Use via getFlags().
@@ -119,3 +118,9 @@
 }
 
 }  // namespace __scudo
+
+#if !SANITIZER_SUPPORTS_WEAK_HOOKS
+SANITIZER_INTERFACE_WEAK_DEF(const char*, __scudo_default_options, void) {
+  return "";
+}
+#endif