Add file modes for opening a file write only

runtime/bin/file_macos.cc

Index: runtime/bin/file_macos.cc
diff --git a/runtime/bin/file_macos.cc b/runtime/bin/file_macos.cc
index 9fc33c5b4936a56b4e36c30b3ac61fc3c8a0c771..ddc19cc029794e899f4c336332d2dcb251ebdcdd 100644
--- a/runtime/bin/file_macos.cc
+++ b/runtime/bin/file_macos.cc
@@ -159,8 +159,13 @@ File* File::Open(const char* name, FileOpenMode mode) {
   }
   int flags = O_RDONLY;
   if ((mode & kWrite) != 0) {
+    ASSERT((mode & kWriteOnly) == 0);
     flags = (O_RDWR | O_CREAT);
   }
+  if ((mode & kWriteOnly) != 0) {
+    ASSERT((mode & kWrite) == 0);
+    flags = (O_WRONLY | O_CREAT);
+  }
   if ((mode & kTruncate) != 0) {
     flags = flags | O_TRUNC;
   }
@@ -169,7 +174,8 @@ File* File::Open(const char* name, FileOpenMode mode) {
     return NULL;
   }
   FDUtils::SetCloseOnExec(fd);
-  if (((mode & kWrite) != 0) && ((mode & kTruncate) == 0)) {
+  if ((((mode & kWrite) != 0) && ((mode & kTruncate) == 0)) ||
+      (((mode & kWriteOnly) != 0) && ((mode & kTruncate) == 0))) {
     int64_t position = lseek(fd, 0, SEEK_END);
     if (position < 0) {
       return NULL;