Add O_APPEND flag for __libc_write_stderr.

For DeathTests, we are testing the output of stderr to check if it is the
death we are expecting. To collect the output, Gtest redirects stderr to
a temporary file. But in __libc_write_stderr in libc_logging.cpp, we are
writing to stderr without a O_APPEND flag, so a new message will overwrite
a previous message.
The above situation makes almost all the DeathTests fail on host. Because
the expected message are always overwritten in host DeathTests. So I add
O_APPEND flag in __libc_write_stderr, which makes all host DeathTests pass.

Change-Id: Ic2f6044fdb181eebe132a6f170b57db43c5c3289

libc/bionic/libc_logging.cpp

@@ -427,7 +427,7 @@ int __libc_format_fd(int fd, const char* format, ...) {
 }
 
 static int __libc_write_stderr(const char* tag, const char* msg) {
-  int fd = TEMP_FAILURE_RETRY(open("/dev/stderr", O_CLOEXEC | O_WRONLY));
+  int fd = TEMP_FAILURE_RETRY(open("/dev/stderr", O_CLOEXEC | O_WRONLY | O_APPEND));
   if (fd == -1) {
     return -1;
   }