Use clog10 not __clog10 in tgmath.h log10 macro.

As a GNU extension, for _GNU_SOURCE glibc's complex.h provides a
clog10 function and tgmath.h supports complex arguments to the log10
macro.  However, tgmath.h uses __clog10 not clog10 in defining the
macro.

There is no namespace reason (ignoring the block-scope namespace
issues that would apply equally to *every* function called by tgmath.h
macros) for using __clog10 here, since this is only for _GNU_SOURCE so
clog10 is always visible when this macro definition is used.
Furthermore, __clog10f128 is not exported, so supporting _Float128 in
tgmath.h implies using clog10 not __clog10 there.  (__clog10 and
clog10 aren't used in libstdc++ either, although that library would
have a good case for using the __clog10 reserved-namespace export: the
standard C++ library includes log10 of a complex number.)  This patch
duly changes the header to use clog10, and enables tests of the macro
for complex arguments.

Tested for x86_64.

	* math/tgmath.h [__USE_GNU] (log10): Use clog10 not __clog10.
	* math/gen-tgmath-tests.py (Tests.add_all_tests): Test log10 for
	complex arguments.