Add conditional code for android's lack of definition of SYS_getdent64.

Fixes issue20307.  No Misc/NEWS entry because frankly this is an
esoteric platform for anyone to be figuring out how to cross compile
CPython for.

Modules/_posixsubprocess.c

@@ -18,6 +18,12 @@
 #include <dirent.h>
 #endif
 
+#if defined(__ANDROID__) && !defined(SYS_getdents64)
+/* Android doesn't expose syscalls, add the definition manually. */
+# include <sys/linux-syscalls.h>
+# define SYS_getdents64  __NR_getdents64
+#endif
+
 #if defined(sun)
 /* readdir64 is used to work around Solaris 9 bug 6395699. */
 # define readdir readdir64