Revert "runtime: fix ARM assembly code in cgocallback"

This reverts CL 479255.

Reason for revert: need to revert CL 392854, and this caused a conflict.

Change-Id: I6cb105c62e51b47de3f652df5f5ee92673a93919
Reviewed-on: https://go-review.googlesource.com/c/go/+/481058
Run-TryBot: Cherry Mui <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Michael Pratt <[email protected]>

Author: cherrymui

src/runtime/asm_arm.s

@@ -633,7 +633,6 @@ TEXT	·cgocallback(SB),NOSPLIT,$12-12
 	// Skip cgocallbackg, just dropm when fn is nil, and frame is the saved g.
 	// It is used to dropm while thread is exiting.
 	MOVW	fn+0(FP), R1
-	CMP	$0, R1
 	B.NE	loadg
 	// Restore the g from frame.
 	MOVW	frame+4(FP), g