Backport the fix to PR28831 plus its follow-up (r280042)

diff --git a/libcxx/lib/CMakeLists.txt b/libcxx/lib/CMakeLists.txt
index afc388e76..4f43f3711 100644
--- a/libcxx/lib/CMakeLists.txt
+++ b/libcxx/lib/CMakeLists.txt
@@ -115,9 +115,9 @@ if ( APPLE AND (LIBCXX_CXX_ABI_LIBNAME STREQUAL "libcxxabi" OR
       "-Wl,-unexported_symbols_list,${CMAKE_CURRENT_SOURCE_DIR}/libc++unexp.exp"
       "/usr/lib/libSystem.B.dylib")
   else()
-    if ( ${CMAKE_OSX_SYSROOT} )
-      list(FIND ${CMAKE_OSX_ARCHITECTURES} "armv7" OSX_HAS_ARMV7)
-      if (OSX_HAS_ARMV7)
+    if (DEFINED CMAKE_OSX_SYSROOT)
+      list(FIND CMAKE_OSX_ARCHITECTURES "armv7" OSX_HAS_ARMV7)
+      if (NOT OSX_HAS_ARMV7 EQUAL -1)
         set(OSX_RE_EXPORT_LINE
           "${CMAKE_OSX_SYSROOT}/usr/lib/libc++abi.dylib"
           "-Wl,-reexported_symbols_list,${CMAKE_CURRENT_SOURCE_DIR}/libc++sjlj-abi.exp")