ada: C++ exception hierarchies: adjust for gnat-llvm

gnat-llvm doesn't support C++ imports, so arrange for the GNAT.CPP*
units to be omitted from gnat-llvm builds.

Drop G++ exception interoperability support from raise-gcc.c, so as to
not require the GNAT.CPP* units that define some of the required
symbols.

Co-Authored-By: Olivier Hainque <hainque@adacore.com>

gcc/ada/ChangeLog:

	* Makefile.rtl (LLVM_BUILD): Define based on LLVM_CONFIG.
	(GNATRTL_NONTASKING_OBJS): Make g-cpp, g-cppstd, and g-cstyin
	conditional on -gcc or -arm EH, and on no LLVM_BUILD.
	* raise-gcc.c (GXX_EH_INTEROP): Define as 0 on gnat-llvm or
	CERT, and 1 otherwise.
	(__gnat_get_cxx_dependent_exception) Omit on !GXX_EH_INTEROP.
	(__gnat_maybe_get_cxx_dependent_exception): Likewise.
	(__gnat_get_cxx_exception_type_info): Likewise.
	(__gnat_obtain_caught_object): Likewise.
	(is_handled_by): Omit eid parameter and G++ interop on
	!GXX_EH_INTEROP.  Adjust callers.