Re-write of the x86 port debug support. This should give a significant
improvement in reliability necessary for breakpoint and single stepping to have a chance, and has been tested in the experimental code and works on FreeBSD and Linux. The code object for a given return address in now located with the help of the GC support function component_ptr_from_pc which can reliably locate the code objects. This uses a forward search of the heap so is rather slow - some caching will be needed for better performance. The stack parsing has been re-written using a backtracking search to improve the chance of finding a valid frame path given that the control changes between C and Lisp conventions. The x86-lra hack is no longer necessary (was unreliable anyway) allowing x86 code to be more compact. The return address is now handled as a SAP which is acceptable because the code doesn't move with CGC, and GENCGC ensures that code objects in the dynamic heap with return addresses on the stack are not moved. However there are known problems under GENCGC when return addresses are move off the stack during some breakpoint operations in which case the code object may move.
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