Rework 128-bit complex multiply and divide.

Rework 128-bit complex multiply and divide.

This function reworks how the complex multiply and divide built-in functions are
done.  Previously we created built-in declarations for doing long double complex
multiply and divide when long double is IEEE 128-bit.  The old code also did not
support __ibm128 complex multiply and divide if long double is IEEE 128-bit.

One of the problems with this approach is that the add_builtin_function would
abort if we used the same name for two different built-in functions.

This code does not create the built-in declaration.  Instead, it uses the
TARGET_MANGLE_DECL_ASSEMBLER_NAME hook to change the name before it is written
out to the assembler file.

2022-08-09   Michael Meissner  <meissner@linux.ibm.com>

gcc/

	* config/rs6000/rs6000-c.cc (rs6000_cpu_cpp_builtins): Set
	building_libgcc.
	* config/rs6000/rs6000.cc (create_complex_muldiv): Delete.
	(init_float128_ieee): Delete code to switch complex multiply and divide
	for long double.
	(complex_multiply_builtin_code): New helper function.
	(complex_divide_builtin_code): Likewise.
	(rs6000_mangle_decl_assembler_name): Add support for mangling the name
	of complex 128-bit multiply and divide built-in functions.
	* config/rs6000/rs6000.opt (building_libgcc): New target variable.