libgcc: Expose the instruction pointer and stack pointer in SEH _Unwind_Backtrace

libgcc: Expose the instruction pointer and stack pointer in SEH _Unwind_Backtrace

Previously, the SEH version of _Unwind_Backtrace did unwind
the stack and call the provided callback function as intended,
but there was little the caller could do within the callback to
actually get any info about that particular level in the unwind.

Set the ra and cfa pointers, which are used by _Unwind_GetIP
and _Unwind_GetCFA, to allow using these functions from the
callacb to inspect the state at each stack frame.

2020-09-08  Martin Storsjö  <martin@martin.st>

	libgcc/
	* unwind-seh.c (_Unwind_Backtrace): Set the ra and cfa pointers
	before calling the callback.