arm64: stacktrace: Factor out common unwind()

Move unwind() to stacktrace/common.h, and as a result
the kernel unwind_next() to asm/stacktrace.h. This allow
reusing unwind() in the implementation of the nVHE HYP
stack unwinder, later in the series.

Signed-off-by: Kalesh Singh <kaleshsingh@google.com>
Reviewed-by: Fuad Tabba <tabba@google.com>
Reviewed-by: Mark Brown <broonie@kernel.org>
Tested-by: Fuad Tabba <tabba@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220726073750.3219117-6-kaleshsingh@google.com