#3 assemble: new directive helping user when saving regs

[Benjamin Gandon]

When adding new saved regs to a "movem.l <regs>,-(sp)",
it's very common that you forget to modify the "movem.l
(sp)+,<regs>" at the end of your subroutine, and the
assembled code will crash when exiting the rubroutine
with rts.

The idea is to help user not to do this mistake at
assembly time. A new directive in Assemble could assign
a tag that would link the first "movem.l" to the
second. Then Assemble would only compare the regs WORD,
check that it's the same for both expressions and raise
an error if it's not.