LoongArch: Fix warnings building libgcc

LoongArch: Fix warnings building libgcc

We are excluding loongarch-opts.h from target libraries, but now struct
loongarch_target and gcc_options are not declared in the target
libraries, causing:

In file included from ../.././gcc/options.h:8,
                 from ../.././gcc/tm.h:49,
                 from ../../../gcc/libgcc/fixed-bit.c:48:
../../../gcc/libgcc/../gcc/config/loongarch/loongarch-opts.h:57:41:
warning: 'struct gcc_options' declared inside parameter list will not
be visible outside of this definition or declaration
   57 |                                  struct gcc_options *opts,
      |                                         ^~~~~~~~~~~

So exclude the declarations referring to the C++ structs as well.

gcc/ChangeLog:

	* config/loongarch/loongarch-opts.h (la_target): Move into #if
	for loongarch-def.h.
	(loongarch_init_target): Likewise.
	(loongarch_config_target): Likewise.
	(loongarch_update_gcc_opt_status): Likewise.