[Assembler] Remove duplicate code

[trylimits]

The assembler code for dabt and timer ISR both use the same code for getting and restoring the context (except the line SUB LR, LR, #4/#8). We could get rid of this code duplication if an assembler function is implemented which gets and restores the context. The c-code would look like this:

address_t* getContext() { ... }

void restoreContext(address_t* context) { ... }

where address_t* points to the stack where the fetched context is stored.

See Bug #68

[ghost]

i'll take care of this as soon as i've refactored the swi handler.