arm64: Make some stacktrace functions private

arm64: Make some stacktrace functions private

Now that open-coded stack unwinds have been converted to
arch_stack_walk(), we no longer need to expose any of unwind_frame(),
walk_stackframe(), or start_backtrace() outside of stacktrace.c.

Make those functions private to stacktrace.c, removing their prototypes
from <asm/stacktrace.h> and marking them static.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Mark Brown <broonie@kernel.org>
Cc: Madhavan T. Venkataraman <madvenka@linux.microsoft.com>
Link: https://lore.kernel.org/r/20211129142849.3056714-10-mark.rutland@arm.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>