use in-fragment mangling to avoid the IBL for OP_blx

Once we've implemented far-through-stub linking, we should switch OP_blx to
use it instead of ibl:

```
/* Unfortunately while there is OP_blx with an immed, OP_bx requires
* indirection through a register. We thus need to swap modes separately,
* but our ISA doesn't support mixing modes in one fragment, making
* a local "blx next_instr" not easy. We have two potential solutions:
* A) Implement far linking through stub's "ldr pc, [pc + 8]" and use
* it for blx. We need to implement that anyway for reachability,
* but as it's not implemented yet, I'm going w/ B) for now.
* B) Pretend this is an indirect branch and use the ibl.
* This is slower so FIXME i#1612: switch to A once we have far links.
*/
```

Except we do need to set the dcontext isa mode: but can do that in-fragment w/
mangling, right? Though what if we get a fault?

Xref #1611 eliminating through-stub linking: but it would leave
load-into-PC as the exit cti, so presumably we could still do the ISA
change as mangling code.