[tsan] Fix weakly imported functions on OS X

On OS X, for weak function (that user can override by providing their own implementation in the main binary), we need extern `"C" SANITIZER_INTERFACE_ATTRIBUTE SANITIZER_WEAK_ATTRIBUTE NOINLINE`.

Fixes a broken test case on OS X, java_symbolization.cc, which uses a weak function __tsan_symbolize_external.

Differential Revision: http://reviews.llvm.org/D14907

llvm-svn: 254298