[libc++] Add missing %link_flags to .sh.cpp test

Without the link flags, the test always fails on Linux. For some reason,
however, it works on Darwin -- which is why it wasn't caught at first.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@366579 91177308-0d34-0410-b5e6-96231b3b80d8

test/libcxx/strings/basic.string/PR42676.sh.cpp

@@ -8,7 +8,7 @@
 
 // Regression test for PR42676.
 
-// RUN: %cxx %flags %s -o %t.exe %compile_flags -D_LIBCPP_HIDE_FROM_ABI_PER_TU
+// RUN: %cxx %flags %s -o %t.exe %compile_flags %link_flags -D_LIBCPP_HIDE_FROM_ABI_PER_TU
 // RUN: %t.exe
 
 #include <memory>