Fix extraneous warning about executable stack.

Fix extraneous warning about executable stack.

PR gold/17578 notes that gold will print a warning about an executable stack
when the -z execstack option is given, even when there is no --warn_execstack
option. The warning is completely useless and unexpected, since the user
explicitly requested an executable stack, and did not even ask for warnings.

This patch fixes that, and adds an extra warning when --warn_execstack
and -z noexecstack are both given and an input file requires an executable
stack.

gold/
	PR gold/17578
	* layout.cc (Layout::layout_gnu_stack): Don't warn when -z execstack
	is given.
	(Layout::create_executable_stack_info): Warn when -z noexecstack is
	given but some inputs require executable stack.