Force use of indirect function calls in inline assembler.

Force use of indirect function calls in inline assembler.

For indirect calls, place function address in a register and call it.
This is less optimal when gcc can inline the code and the destination
address is known at compile time.  However, older gcc compilers don't
do as well with inlining, and they then mess up the code generation.
There doesn't seem to be a way to tell gcc how to emit the code
correctly for both immediate addresses and register addresses, so fall
back to a safe way.

Also, reduce params to stack_hop to avoid register assignment issues.