ARC: [arcompact] entry.S: Improve early return from exception

The requirement is to
 - Reenable Exceptions (AE cleared)
 - Reenable Interrupts (E1/E2 set)

We need to do wiggle these bits into ERSTATUS and call RTIE.

Prev version used the pre-exception STATUS32 as starting point for what
goes into ERSTATUS. This required explicit fixups of U/DE/L bits.

Instead, use the current (in-exception) STATUS32 as starting point.
Being in exception handler U/DE/L can be safely assumed to be correct.
Only AE/E1/E2 need to be fixed.

So the new implementation is slightly better
 -Avoids read form memory
 -Is 4 bytes smaller for the typical 1 level of intr configuration
 -Depicts the semantics more clearly
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>

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

@@ -110,13 +110,12 @@
 
 .macro FAKE_RET_FROM_EXCPN
 
-	ld  r9, [sp, PT_status32]
-	bic r9, r9, (STATUS_U_MASK|STATUS_DE_MASK)
-	bset  r9, r9, STATUS_L_BIT
+	lr	r9, [status32]
+	bclr	r9, r9, STATUS_AE_BIT
+	or	r9, r9, (STATUS_E1_MASK|STATUS_E2_MASK)
 	sr	r9, [erstatus]
 	mov	r9, 55f
 	sr	r9, [eret]
-
 	rtie
 55:
 .endm

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

@@ -23,11 +23,13 @@
 #define STATUS_E2_BIT		2	/* Int 2 enable */
 #define STATUS_A1_BIT		3	/* Int 1 active */
 #define STATUS_A2_BIT		4	/* Int 2 active */
+#define STATUS_AE_BIT		5	/* Exception active */
 
 #define STATUS_E1_MASK		(1<<STATUS_E1_BIT)
 #define STATUS_E2_MASK		(1<<STATUS_E2_BIT)
 #define STATUS_A1_MASK		(1<<STATUS_A1_BIT)
 #define STATUS_A2_MASK		(1<<STATUS_A2_BIT)
+#define STATUS_AE_MASK		(1<<STATUS_AE_BIT)
 #define STATUS_IE_MASK		(STATUS_E1_MASK | STATUS_E2_MASK)
 
 /* Other Interrupt Handling related Aux regs */