arch/m68k-amiga: gencall - fix clobbered return register issue

arch/m68k-amiga: gencall - fix clobbered return register issue

Fix an issue where, occasionally, code of the form:

if (LibCall1(d0_arg) == constant) {
   LibCall2(a0_arg);
}

generated assembly where LibCall2() clobbered the return value from
LibCall1(), and then the return value in d0 was then checked.

The new generated libcall.h makes slightly tighter assembly code,
but caused a gcc optimization failure for -Os in mathieeedoubtrans.library

(The former is good, the latter has been worked around by
 compiling that library with -O1 on m68k)

The previous workaround for Prefs/Printer has been removed.

Signed-off-by: Jason S. McMullan <jason.mcmullan@gmail.com>
git-svn-id: https://svn.aros.org/svn/aros/trunk/AROS@47425 fb15a70f-31f2-0310-bbcc-cdcc74a49acc