Arrange for arm fork_trampoline() to return to userland via the standard
swi_exit code in exception.S instead of having its own inline expansion
of the DO_AST and PULLFRAME macros.  That means that now all references
to the PUSH/PULLFRAME and DO_AST macros are localized to exception.S,
so move the macros themselves into there and remove them from asmacros.h