Add -Wstrict-null-sentinel to gdbsupport/warning.m4

Add -Wstrict-null-sentinel to gdbsupport/warning.m4

Commit 85f0dd3ce ("[gdb] Fix -Wstrict-null-sentinel warnings") fixed
some violations of -Wstrict-null-sentinel.  If we want to enforce this
warning, I think we should enable it in our warning.m4 file.

gdbsupport/ChangeLog:

	* warning.m4: Add -Wstrict-null-sentinel.
	* configure: Re-generate.

gdbserver/ChangeLog:

	* configure: Re-generate.

gdb/ChangeLog:

	* configure: Re-generate.