stdlib/tst-strtod-round.c: Fix build on ARM

stdlib/tst-strtod-round.c: Fix build on ARM

Building this test on ARM fails because the prototypes for the long
double variants of the math functions are unavailable.

Add an additional include guard to math.h that enables long double math
function declarations if _LIBC_TEST is defined and define _LIBC_TEST in
stdlib/tst-strtod-round.c.

ChangeLog:

2014-09-30  Will Newton  <will.newton@linaro.org>

	* math/math.h: Define long double math functions if
	_LIBC_TEST is defined.
	* stdlib/tst-strtod-round.c: Define _LIBC_TEST.