compiler/arm/call: Fix source locations in the debugger.

  * It turns out that the debugger is spectacularly picky about
the exact position of noted locations.  To the point where a mere
twelve-octet displacement of the noted location of a function
return will throw off its ability to find source forms.

  * Fix, by noting the location of return locations in
DEFAULT-UNKNOWN-VALUE to before the first instruction emitted.