arc: Fix (u)maddhisi patterns

arc: Fix (u)maddhisi patterns

Rework the (u)maddhisi4 patterns and use VMAC2H(U) instruction instead
of the 64bit MAC(U) instruction.
This fixes the next execute.exp failures:
     arith-rand-ll.c   -O2  execution test
     arith-rand-ll.c   -O3  execution test
     pr78726.c   -O2  execution test
     pr78726.c   -O3  execution test

gcc/
2021-06-09  Claudiu Zissulescu  <claziss@synopsys.com>

	* config/arc/arc.md (maddhisi4): Use VMAC2H instruction.
	(machi): New pattern.
	(umaddhisi4): Use VMAC2HU instruction.
	(umachi): New pattern.

Signed-off-by: Claudiu Zissulescu <claziss@synopsys.com>