ARCv2: entry: simplify return to Delay Slot via interrupt

ARCv2: entry: simplify return to Delay Slot via interrupt

Commit 4255b07f2c9c43540 ("ARCv2: STAR 9000793984: Handle return
from intr to Delay Slot") involved a complex 2 staged trampoline.

Apparently this can be greatly simplified by returning from pure
kernel mode (iso interrupt) so drop to pure kernel mdoe and execute
the normal exception return path.

Testing this was a bit of challenge as return from interrupt is rarely
executed now after commit 4de0e52867d83105767 ("ARCv2: STAR 9000814690:
Really Re-enable interrupts to avoid deadlocks"). That fix is necessary
evil and pct interrupts etc do exercise intr return path.
Anyhow after a revert of above in my local test setup I was able to hit
this case and verify the patch works.

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