syscall: Add call gate for get_environ_ptr

Now that the environment strings are stored as an array of strings,
they can be passed from the application via char **environ, which
is really defined as a function call to get_environ_ptr().

This works as is for flat build, but protected mode and kernel mode
require a call gate, which is added here.

include/sys/syscall_lookup.h

@@ -353,6 +353,7 @@ SYSCALL_LOOKUP(futimens,                   2)
 /* The following are defined only if environment variables are supported */
 
 #ifndef CONFIG_DISABLE_ENVIRON
+  SYSCALL_LOOKUP(get_environ_ptr,          0)
   SYSCALL_LOOKUP(clearenv,                 0)
   SYSCALL_LOOKUP(getenv,                   1)
   SYSCALL_LOOKUP(putenv,                   1)

syscall/syscall.csv

@@ -35,6 +35,7 @@
 "fsync","unistd.h","!defined(CONFIG_DISABLE_MOUNTPOINT)","int","int"
 "ftruncate","unistd.h","!defined(CONFIG_DISABLE_MOUNTPOINT)","int","int","off_t"
 "futimens","sys/stat.h","","int","int","const struct timespec [2]|FAR const struct timespec *"
+"get_environ_ptr","stdlib.h","!defined(CONFIG_DISABLE_ENVIRON)","FAR char **"
 "getenv","stdlib.h","!defined(CONFIG_DISABLE_ENVIRON)","FAR char *","FAR const char *"
 "getgid","unistd.h","defined(CONFIG_SCHED_USER_IDENTITY)","gid_t"
 "gethostname","unistd.h","","int","FAR char *","size_t"