Implement <spawn.h>.

As described here:

  http://pubs.opengroup.org/onlinepubs/9699919799/functions/posix_spawn.html

And here:

  http://man7.org/linux/man-pages/man3/posix_spawn.3.html

Bug: N/A (but mentioned in my inbox since 2013)
Test: ran tests
Change-Id: I0b27b2919b660779e3bd8a25fb429527c16dc621
diff --git a/libc/Android.bp b/libc/Android.bp
index cd01efd..a142f4e 100644
--- a/libc/Android.bp
+++ b/libc/Android.bp
@@ -1530,6 +1530,7 @@
         "bionic/sigwait.cpp",
         "bionic/sigwaitinfo.cpp",
         "bionic/socket.cpp",
+        "bionic/spawn.cpp",
         "bionic/stat.cpp",
         "bionic/statvfs.cpp",
         "bionic/stdlib_l.cpp",