Check if MemorySanitizer is used on x86_64 Linux.

According to the MemorySanitizer documentation, it is only compatible with 64bit
Linux at the moment.

cmake/FindMSan.cmake

@@ -31,10 +31,25 @@ set(FLAG_CANDIDATES
 
 include(sanitize-helpers)
 
-sanitizer_check_compiler_flags("${FLAG_CANDIDATES}" "MemorySanitizer" "MSan")
+if (SANITIZE_MEMORY)
+    if (NOT ${CMAKE_SYSTEM_NAME} STREQUAL "Linux")
+        message(WARNING "MemorySanitizer disabled for target ${TARGET} because "
+            "MemorySanitizer is supported for Linux systems only.")
+        set(SANITIZE_MEMORY Off CACHE BOOL
+            "Enable MemorySanitizer for sanitized targets." FORCE)
+    elseif (NOT ${CMAKE_SIZEOF_VOID_P} EQUAL 8)
+        message(WARNING "MemorySanitizer disabled for target ${TARGET} because "
+            "MemorySanitizer is supported for 64bit systems only.")
+        set(SANITIZE_MEMORY Off CACHE BOOL
+            "Enable MemorySanitizer for sanitized targets." FORCE)
+    else ()
+        sanitizer_check_compiler_flags("${FLAG_CANDIDATES}" "MemorySanitizer"
+            "MSan")
+    endif ()
+endif ()
 
 function (add_sanitize_memory TARGET)
-    if (NOT SANITIZE_THREAD)
+    if (NOT SANITIZE_MEMORY)
         return()
     endif ()