m68k: add MSP detection support for stack pointer swap helpers

m68k: add MSP detection support for stack pointer swap helpers

On m68k there are two varities of stack pointers: USP with SSP or ISP/MSP.

Only the 68020/30/40 support the MSP register the stack swap helpers don't
support this feature.

This patch adds this support, as well as comments to CPUM68KState to
make it clear how stacks are handled

Signed-off-by: Lucien Murray-Pitts <lucienmp.qemu@gmail.com>
Signed-off-by: BALATON Zoltan <balaton@eik.bme.hu>
Message-Id: <c61ad2d8b39f3b03b431819b6bf602a1c332b921.1612137712.git.balaton@eik.bme.hu>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>