UPSTREAM: arm64: unexport walk_stackframe

The walk_stackframe functions is architecture-specific, with a varying
prototype, and common code should not use it directly. None of its
current users can be built as modules. With THREAD_INFO_IN_TASK, users
will also need to hold a stack reference before calling it.

There's no reason for it to be exported, and it's very easy to misuse,
so unexport it for now.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
(cherry picked from commit 2020a5ae7c8c2c8504565004915017507b135c63)

Change-Id: If09c303ccd98dcc030e20a16a2eab5bf7bb54e4a
Signed-off-by: Greg Hackmann <ghackmann@google.com>