powerpc: Restrict xssqrtqp operands to Vector Registers (bug 21941)

POWER ISA 3.0 introduces the xssqrtqp instructions, which expects
operands to be in Vector Registers (Altivec/VMX), even though this
instruction belongs to the Vector-Scalar Instruction Set.

In GCC's Extended Assembly for POWER, the 'wq' register constraint is
provided for use with IEEE 754 128-bit floating-point values.  However,
this constraint does not limit the register allocation to Vector
Registers (Altivec/VMX) and could assign a Vector-Scalar Register (VSX)
to the operands of the instruction.

This patch changes the register constraint used in sqrtf128 from 'wq' to
'v', in order to request a Vector Register (Altivec/VMX) for use with
the xssqrtqp instruction.

Tested for powerpc64le and --with-cpu=power9.

	[BZ #21941]
	* sysdeps/powerpc/fpu/math_private.h (__ieee754_sqrtf128): Since
	xssqrtqp requires operands to be in Vector Registers
	(Altivec/VMX), replace the register constraint 'wq' with 'v'.
	* sysdeps/powerpc/powerpc64le/power9/fpu/e_sqrtf128.c
	(__ieee754_sqrtf128): Likewise.