The current assembler will report error when the negative offset is in
the range of [-256, 0) and is not the multiples of 4/8.

The fix introduces C_NSAUTO_8, C_NSAUTO_4 and C_NAUTO4K. C_NPAUTO
includes C_NSAUTO_8 instead of C_NSAUTO, C_NAUTO4K includes C_NSAUTO_8,
C_NSAUTO_4 and C_NSAUTO. So that assembler will encode the negative offset
that is greater than -4095 and is not the multiples of 4/8 as two instructions.

Add the test cases.

Fixed #24471

Change-Id: I42f34e3b8a9fc52c9e8b41504294271aafade639
Reviewed-on: https://go-review.googlesource.com/102635
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>