Use C11 CMPLX* macros in libm tests.

Use C11 CMPLX* macros in libm tests.

libm-test.inc has a macro BUILD_COMPLEX to construct a complex number
with given real and imaginary parts while allowing properly for signed
zeroes, infinities and NaNs (which don't work properly with a simple
real + I * imag, in the absence of compiler support for imaginary
types), using assignment to __real__ and __imag__ parts of the number.
C11 defines CMPLX* macros for this purpose, which GCC 4.7 and above
provide suitable built-in functions for.  This patch redefines
BUILD_COMPLEX in terms of the standard macros.

Tested for x86_64 and x86.

	* math/libm-test.inc (BUILD_COMPLEX): Remove macro.
	* math/test-double.h (BUILD_COMPLEX): New macro.
	* math/test-float.h (BUILD_COMPLEX): Likewise.
	* math/test-ldouble.h (BUILD_COMPLEX): Likewise.