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

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>