arc: generate mac(u) insn instead of macd(u) when destination is accl

arc: generate mac(u) insn instead of macd(u) when destination is accl

Generate MAC(U) instruction instead of MACD(U) when the destination
register is already choosen as ACCL register.

gcc/
2020-12-29  Claudiu Zissulescu  <claziss@synopsys.com>

	* config/arc/arc.md (maddsidi4_split): Skip macd gen, use mac insn
	instead.
	(macd): Update register letters.
	(umaddsidi4_split): Skip macdu gen, use macu insn instead.
	(macdu): Update register letters.

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