Implement unwind rules for non-Intel architectures

To support DWARF-based stack unwinding, a platform needs a few things,

• Valid unwind records in stg_stop_thread (defined in rts/StgStartup.cmm)
• Support in the native code generator (by implementing the generateUnwindTable field of NcgImpl)
• A mapping of register numbers to DWARF register numbers (see dwarfRegNo in nativeGhc/Dwarf/Constants.hs)
• For unwinding support in the runtime system:
• Unwinding support in libdw
• a set_initial_registers implementation (see rts/Libdw.c)

Currently the only platforms that has all of these implemented is x86_64 and i386. Someone should pick up the other platforms.

Trac metadata

Trac field	Value
Version	8.0.1
Type	Task
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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