Fix clog10 (-0 +/- 0i) (bug 16362).

This patch fixes the imaginary part of clog10 (-0 +/- 0i), which
should be +/-pi / log(10) by analogy with clog (the functions were
wrongly returning a result with imaginary part +/-pi, same as for
clog, and the tests matched the incorrect result, though both
functions and tests were correct for the similar case of clog10 (-inf
+/- 0i)).  Tested x86_64 and x86.

	[BZ #16362]
	* math/s_clog10.c (M_PI_LOG10E): New macro.
	(__clog10): Use M_PI_LOG10E instead of M_PI when real and
	imaginary parts are 0.
	* math/s_clog10f.c (M_PI_LOG10Ef): New macro.
	(__clog10f): Use M_PI_LOG10Ef instead of M_PI when real and
	imaginary parts are 0.
	* math/s_clog10l.c (M_PI_LOG10El): New macro.
	(__clog10l): Use M_PI_LOG10El instead of M_PIl when real and
	imaginary parts are 0.
	* math/libm-test.inc (clog10_test_data): Update expected results
	for when real and imaginary parts are 0.