Remove ldbl-128ibm variants of complex math functions.

sysdeps/ieee754/ldbl-128ibm has its own versions of cprojl, ctanhl and
ctanl.

Having its own versions, where otherwise the math/ copies are
generally used for all floating-point formats, means they are liable
to get out of sync and not benefit from bug fixes to the generic
versions.  The substantive differences (not arising from getting out
of sync and slightly different fixes for the same issues) are: long
double compat handling (also done in the ldbl-opt versions, so doesn't
require special versions for ldbl-128ibm); handling of LDBL_EPSILON
(conditionally undefined and redefined in other math/ implementations,
so doesn't justify a special version), and:

      /* __gcc_qmul does not respect -0.0 so we need the following fixup.  */
      if ((__real__ res == 0.0L) && (__real__ x == 0.0L))
        __real__ res = __real__ x;

      if ((__real__ res == 0.0L) && (__imag__ x == 0.0L))
        __imag__ res = __imag__ x;

But if that statement about __gcc_qmul was ever true for an old
version of that libgcc function, it's not the case for any GCC version
now supported to build glibc; there's explicit logic early in that
function (and similarly in __gcc_qdiv) to return an appropriately
signed zero if the product of the high parts is zero.  So this patch
adds the special LDBL_EPSILON handling to the generic functions and
removes the ldbl-128ibm versions.

Tested for powerpc32 (compared test-ldouble.out before and after the
changes; there are slight changes to results for ctanl / ctanhl,
arising from divergence of the implementations, but nothing that
affects the overall nature of the issues shown by the testsuite, and
in particular nothing related to signs of zero resutls).

	* math/s_ctanhl.c [LDBL_MANT_DIG == 106] (LDBL_EPSILON): Undefine
	and redefine.
	* math/s_ctanl.c [LDBL_MANT_DIG == 106] (LDBL_EPSILON): Undefine
	and redefine.
	* sysdeps/ieee754/ldbl-128ibm/s_cprojl.c: Remove file.
	* sysdeps/ieee754/ldbl-128ibm/s_ctanhl.c: Likewise.
	* sysdeps/ieee754/ldbl-128ibm/s_ctanl.c: Likewise.