Disable LRA for MMIX.

Disable LRA for MMIX.

Since the change r14-383-gfaf8bea79b6256 "Enable LRA on
several ports", mmix has been broken building libstdc++-v3:

libtool: compile: /obj/./gcc/xgcc -shared-libgcc -B/obj/./gcc
-nostdinc++ -L/obj/mmix/libstdc++-v3/src
-L/obj/mmix/libstdc++-v3/src/.libs
-L/obj/mmix/libstdc++-v3/libsupc++/.libs -nostdinc -B/obj/mmix/newlib/
-isystem /obj/mmix/newlib/targ-include -isystem
/gcctop/newlib/libc/include -B/obj/mmix/libgloss/mmix
-L/obj/mmix/libgloss/libnosys -L/gcctop/libgloss/mmix
-B/home/hp/tmp/mmix230811-00/pre/mmix/bin/
-B/home/hp/tmp/mmix230811-00/pre/mmix/lib/ -isystem
/home/hp/tmp/mmix230811-00/pre/mmix/include -isystem
/home/hp/tmp/mmix230811-00/pre/mmix/sys-include
-I/gcctop/libstdc++-v3/../libgcc -I/obj/mmix/libstdc++-v3/include/mmix
-I/obj/mmix/libstdc++-v3/include -I/gcctop/libstdc++-v3/libsupc++
-fno-implicit-templates -Wall -Wextra -Wwrite-strings -Wcast-qual
-Wabi=2 -fdiagnostics-show-location=once -ffunction-sections
-fdata-sections -frandom-seed=eh_type.lo -g -O2 -c
/gcctop/libstdc++-v3/libsupc++/eh_type.cc -o eh_type.o
/gcctop/libstdc++-v3/libsupc++/eh_terminate.cc: In function 'void
__cxxabiv1::__terminate(std::terminate_handler)':
/gcctop/libstdc++-v3/libsupc++/eh_terminate.cc:53:1: error: unable to
generate reloads for:

   53 | }
      | ^
(insn 31 36 44 4 (parallel [
            (unspec_volatile [
                    (plus:DI (reg/f:DI 253 $253)
                        (const_int 24 [0x18]))
                ] 1)
            (clobber (reg:DI 275))
            (clobber (reg:DI 259 rJ))
        ]) "/gcctop/libstdc++-v3/libsupc++/eh_terminate.cc":51:3
 discrim 1 63 {*nonlocal_goto_receiver_expanded}
     (expr_list:REG_UNUSED (reg:DI 275)
        (expr_list:REG_UNUSED (reg:DI 259 rJ)
            (nil))))
during RTL pass: reload
/gcctop/libstdc++-v3/libsupc++/eh_terminate.cc:53:1:
internal compiler error: in curr_insn_transform, at lra-constraints.cc:4281

This commit temporarily reverts the MMIX part of
r14-383-gfaf8bea79b6256 back to reload.

	* config/mmix/mmix.cc: Disable LRA for MMIX.