ARC: entry.S: micro-optimize Trap handler

Elide the need to re-read ECR in Trap handler by ensuring that
EXCEPTION_PROLOGUE does that at the very end just before returning
to Trap handler

ARCv2 EXCEPTION_PROLOGUE already did that, so same for ARcompact and the
common trap handler adjusted to use cached ECR
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>

arch/arc/include/asm/entry-compact.h

@@ -181,8 +181,6 @@
 	/* ARC700 doesn't provide auto-stack switching */
 	SWITCH_TO_KERNEL_STK
 
-	lr	r9, [ecr]
-	st      r9, [sp, 8]    /* ECR */
 	st      r0, [sp, 4]    /* orig_r0, needed only for sys calls */
 
 	/* Restore r9 used to code the early prologue */
@@ -198,6 +196,9 @@
 	PUSHAX	lp_end
 	PUSHAX	lp_start
 	PUSHAX	erbta
+
+	lr	r9, [ecr]
+	st      r9, [sp, PT_event]    /* EV_Trap expects r9 to have ECR */
 .endm
 
 /*--------------------------------------------------------------

arch/arc/kernel/entry.S

@@ -258,8 +258,8 @@ ENTRY(EV_Trap)
 	EXCEPTION_PROLOGUE
 
 	;============ TRAP 1   :breakpoints
-	lr     r10, [ecr]
-	bmsk.f 0, r10, 7
+	; Check ECR for trap with arg (PROLOGUE ensures r9 has ECR)
+	bmsk.f 0, r9, 7
 	bnz    trap_with_param
 
 	;============ TRAP  (no param): syscall top level