alpha/clone.S: Invoke .set noat/.set at around explicit uses of $at

On Alpha, the register $at is, by default, reserved for use by the
assembler, in the expansion of pseudo-instructions.  It's also used
by the special calling convention for _mcount.  We get warnings from
Alpha clone.S because the code to call _mcount isn't properly marked
up to tell the assembler not to use $at itself.

	* sysdeps/unix/sysv/linux/alpha/clone.s (__clone): Wrap manual
	uses of $at in .set noat / .set at.