When moving a local variable from the stack to static storage, the
procedure is no longer safe to be called recursively or concurrently
from multiple threads.  Thus generate a warning when this is done.
Also double the default limit for switching from stack to static.

Regtested on x86_64-pc-linux-gnu.

gcc/fortran/ChangeLog:

2019-08-11  Janne Blomqvist  <jb@gcc.gnu.org>

	PR fortran/91413
	* invoke.texi (-fmax-stack-var-size): Document increased default.
	* options.c (gfc_post_options): Increase default stack var size to
	65536 bytes.
	* trans-decl.c (gfc_finish_var_decl): Generate warning when local
	array moved to static storage.

From-SVN: r274264