ARC: Remove explicit passing around of ECR

With ECR now part of pt_regs

* No need to propagate from lowest asm handlers as arg
* No need to save it in tsk->thread.cause_code
* Avoid bit chopping to access the bit-fields

More code consolidation, cleanup

Signed-off-by: Vineet Gupta <vgupta@synopsys.com>

arch/arc/include/asm/processor.h

@@ -29,7 +29,6 @@ struct thread_struct {
 	unsigned long ksp;	/* kernel mode stack pointer */
 	unsigned long callee_reg;	/* pointer to callee regs */
 	unsigned long fault_address;	/* dbls as brkpt holder as well */
-	unsigned long cause_code;	/* Exception Cause Code (ECR) */
 #ifdef CONFIG_ARC_FPU_SAVE_RESTORE
 	struct arc_fpu fpu;
 #endif