Avoid signed shift overflow in pow (bug 21309).

As noted in bug 21309, dbl-64/e_pow.c contains signed int shifts that,
although the shift count is in the range [0, 31], shift bits into and
beyond the sign bit and so are undefined in ISO C.  Although this is
defined in GNU C, this patch from the bug cleans up the code to avoid
those shifts.

Tested for x86_64.

	[BZ #21309]
	* sysdeps/ieee754/dbl-64/e_pow.c (checkint): Make m and n
	unsigned.