ArmPkg/DefaultExceptionHandlerLib: walk call stack unconditionally

ArmPkg/DefaultExceptionHandlerLib: walk call stack unconditionally

Currently, we only attempt to walk the call stack and print a backtrace
if the program counter refers to a location covered by a PE/COFF image.
However, regardless of the value of PC, the frame pointer may still have
a meaningful value, and so we can still produce the remainder of the
backtrace.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>