arm64: stacktrace: Implement arch_stack_walk_reliable()

Add arch_stack_walk_reliable(), which will be used during kernel live
patching to detect when threads have completed executing old versions of
functions.

Note that arch_stack_walk_reliable() only needs to guarantee that it
returns an error code when it cannot provide a reliable stacktrace. It
is not required to provide a reliable stacktrace in all scenarios so
long as it returns said error code.

At present we can only reliably unwind up to an exception boundary. In
future we should be able to improve this with additional data from the
compiler (e.g. sframe).

Signed-off-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20250320171559.3423224-2-song@kernel.org
[ Mark: Simplify logic, clarify commit message ]
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Andrea della Porta <andrea.porta@suse.com>
Cc: Breno Leitao <leitao@debian.org>
Cc: Josh Poimboeuf <jpoimboe@kernel.org>
Cc: Miroslav Benes <mbenes@suse.cz>
Cc: Petr Mladek <pmladek@suse.com>
Cc: Song Liu <song@kernel.org>
Cc: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20250521111000.2237470-3-mark.rutland@arm.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

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