ARCv2: build: use mcpu=hs38 iso generic mcpu=archs

helps gcc with better instruction selections such as 64-bit multiply MPYD

before
------
82c34b58 <sched_clock>:
82c34b58:	ld	r2,[0x83068d00]
82c34b60:	add_s	r2,r2,0x7530
82c34b66:	mov_s	r0,0x989680
82c34b6c:	mpymu	r5,r2,r0
82c34b70:	mpy	r4,r2,r0
82c34b74:	mov_s	r0,r4
82c34b76:	j_s.d	[blink]
82c34b78:	mov_s	r1,r5
82c34b7a:	nop_s

after
------
82c34b7c <sched_clock>:
82c34b7c:	ld	r0,[0x83064d00]
82c34b84:	add_s	r0,r0,0x7530
82c34b8a:	mpydu	r0,r0,0x989680
82c34b92:	j_s	[blink]
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>

arch/arc/Makefile

@@ -18,7 +18,7 @@ KBUILD_DEFCONFIG := nsim_700_defconfig
 
 cflags-y	+= -fno-common -pipe -fno-builtin -mmedium-calls -D__linux__
 cflags-$(CONFIG_ISA_ARCOMPACT)	+= -mA7
-cflags-$(CONFIG_ISA_ARCV2)	+= -mcpu=archs
+cflags-$(CONFIG_ISA_ARCV2)	+= -mcpu=hs38
 
 is_700 = $(shell $(CC) -dM -E - < /dev/null | grep -q "ARC700" && echo 1 || echo 0)