Add intial plaform support for Native Client (NaCl)

The initial port to NaCl was done in the webports
repository:
https://chromium.googlesource.com/webports.git/+/master/ports/grpc/

In order to be buildable under NaCl outside the
webports repo there are several further changes
that will be needed:
1. Make AF_UNIX sockets optional
2. Make IP_PKTINFO optional
3. Make vector I/O (readv/writev) optional

include/grpc/impl/codegen/port_platform.h

@@ -247,8 +247,41 @@
 #else /* _LP64 */
 #define GPR_ARCH_32 1
 #endif /* _LP64 */
+#elif defined(__native_client__)
+#define GPR_PLATFORM_STRING "nacl"
+#ifndef _BSD_SOURCE
+#define _BSD_SOURCE
+#endif
+#ifndef _DEFAULT_SOURCE
+#define _DEFAULT_SOURCE
+#endif
+#ifndef _GNU_SOURCE
+#define _GNU_SOURCE
+#endif
+#define GPR_CPU_POSIX 1
+#define GPR_GCC_ATOMIC 1
+#define GPR_GCC_TLS 1
+#define GPR_POSIX_LOG 1
+#define GPR_POSIX_MULTIPOLL_WITH_POLL 1
+#define GPR_POSIX_WAKEUP_FD 1
+#define GPR_POSIX_NO_SPECIAL_WAKEUP_FD 1
+#define GPR_POSIX_SOCKET 1
+#define GPR_POSIX_SOCKETADDR 1
+#define GPR_POSIX_SOCKETUTILS 1
+#define GPR_POSIX_ENV 1
+#define GPR_POSIX_FILE 1
+#define GPR_POSIX_STRING 1
+#define GPR_POSIX_SUBPROCESS 1
+#define GPR_POSIX_SYNC 1
+#define GPR_POSIX_TIME 1
+#define GPR_GETPID_IN_UNISTD_H 1
+#ifdef _LP64
+#define GPR_ARCH_64 1
+#else /* _LP64 */
+#define GPR_ARCH_32 1
+#endif /* _LP64 */
 #else
-#error Could not auto-detect platform
+#error "Could not auto-detect platform"
 #endif
 #endif /* GPR_NO_AUTODETECT_PLATFORM */