Consolidate non cancellable waitpid call

This patch consolidates all the non cancellable waitpid calls to use
the __waitpid_nocancel identifier.  For non cancellable targets it will
be just a macro to call the default respective symbol while on Linux
will be a internal one.

Checked on x86_64-linux-gnu, x86_64-linux-gnu-x32, and i686-linux-gnu.

	* libio/ioopen.c (_IO_waitpid): Replace waitpid_not_cancel with
	__waitpid_nocancel.
	* sysdeps/generic/not-cancel.h (waitpid_not_cancel): Remove macro.
	(__waitpid_nocancel): New macro.
	* sysdeps/unix/sysv/linux/not-cancel.h (waitpid_not_cancel): Remove
	macro.
	(__waitpid_nocancel): Replace macro with a function.
	* sysdeps/unix/sysv/linux/waitpid.c (__waitpid_nocancel): New
	function.