[ARC] Save mlo/mhi registers when ISR.

ARC600 when configured with mul64 instructions uses mlo and mhi
registers to store the 64 result of the multiplication. In the ARC600
ISA documentation we have the next register configuration when ARC600
is configured only with mul64 extension:

Register | Name | Use
---------+------+------------------------------------
r57      | mlo  | Multiply low 32 bits, read only
r58      | mmid | Multiply middle 32 bits, read only
r59      | mhi  | Multiply high 32 bits, read only
-----------------------------------------------------

When used for Co-existence configurations we have for mul64 the next
registers used:

Register | Name | Use
---------+------+------------------------------------
r58      | mlo  | Multiply low 32 bits, read only
r59      | mhi  | Multiply high 32 bits, read only
-----------------------------------------------------

Note that mlo/mhi assignment doesn't swap when bigendian CPU
configuration is used.

The compiler will always use r58 for mlo, regardless of the
configuration choosen to ensure mlo/mhi correct splitting. Fixing mlo
to the right register number is done at assembly time. The dwarf info
is also notified via DBX_... macro. Both mlo/mhi registers needs to
saved when ISR happens using a custom sequence.

gcc/
xxxx-xx-xx  Claudiu Zissulescu  <claziss@synopsys.com>

	* config/arc/arc-protos.h (gen_mlo): Remove.
	(gen_mhi): Likewise.
	* config/arc/arc.c (AUX_MULHI): Define.
	(arc_must_save_reister): Special handling for r58/59.
	(arc_compute_frame_size): Consider mlo/mhi registers.
	(arc_save_callee_saves): Emit fp/sp move only when emit_move
	paramter is true.
	(arc_conditional_register_usage): Remove TARGET_BIG_ENDIAN from
	mlo/mhi name selection.
	(arc_restore_callee_saves): Don't early restore blink when ISR.
	(arc_expand_prologue): Add mlo/mhi saving.
	(arc_expand_epilogue): Add mlo/mhi restoring.
	(gen_mlo): Remove.
	(gen_mhi): Remove.
	* config/arc/arc.h (DBX_REGISTER_NUMBER): Correct register
	numbering when MUL64 option is used.
	(DWARF2_FRAME_REG_OUT): Define.
	* config/arc/arc.md (arc600_stall): New pattern.
	(VUNSPEC_ARC_ARC600_STALL): Define.
	(mulsi64): Use correct mlo/mhi registers.
	(mulsi_600): Clean it up.
	* config/arc/predicates.md (mlo_operand): Remove any dependency on
	TARGET_BIG_ENDIAN.
	(mhi_operand): Likewise.

testsuite/
xxxx-xx-xx  Claudiu Zissulescu  <claziss@synopsys.com>

	* gcc.target/arc/interrupt-6.c: Update test.