Fix ldbl-128 powl sign of result in overflow / underflow cases (bug 17097).

This patch fixes bug 17097, ldbl-128 powl producing overflowing /
underflowing results with positive sign when the result should have
been negative.  This was shown up by the tests in non-default rounding
modes added by my patch for bug 16315, but isn't actually limited to
non-default rounding modes: rather, when rounding to nearest the
wrappers produced a result with the correct sign and so always hid the
bug unless -lieee was used to disable the wrappers.  The problem is
that in the cases where Y is large enough that the result overflows or
underflows for X not very close to 1, but not large enough to overflow
or underflow for all X != +/- 1 (in the latter case Y is always an
even integer), a positive overflowing / underflowing result is always
returned, rather than one with the correct sign.  This patch moves the
relevant part of computation of the sign earlier and returns a result
of the correct sign.

Tested for mips64.

	[BZ #17097]
	* sysdeps/ieee754/ldbl-128/e_powl.c (__ieee754_powl): Return
	result with correct sign in case of exponents that produce
	overflow except for X very close to 1.