arm64: stacktrace: report source of unwind data

When analysing a stacktrace it can be useful to know where an unwound PC
came from, as in some situations certain sources may be suspect or known
to be unreliable. In future it would also be useful to track this so
that certain unwind steps can be performed in a stateful manner. For
example when unwinding across an exception boundary, we'd ideally unwind
pt_regs::pc, then pt_regs::lr, then the next frame record.

This patch adds an enumerated set of unwind sources, tracks this during
the unwind, and updates dump_backtrace() to log these for interesting
unwind steps.

The interesting sources recorded are:

 "C" - the PC came from the caller of an unwind function.
 "T" - the PC came from thread_saved_pc() for a blocked task.
 "P" - the PC came from a pt_regs::pc.
 "U" - the PC came from an unknown source (indicates an unwinder error).

... with nothing recorded when the PC came from a frame_record::pc as
this is the vastly common case and logging this would make it difficult
to spot the more interesting cases.

For example, when triggering a backtrace via magic-sysrq + L, the CPU
handling the sysrq will have a backtrace whose first element is the
caller (C) of dump_backtrace():

| Call trace:
|  show_stack+0x18/0x30 (C)
|  dump_stack_lvl+0x60/0x80
|  dump_stack+0x18/0x24
|  nmi_cpu_backtrace+0xfc/0x140
| ...

... and other CPUs will have a backtrace whose first element is their
pt_regs::pc (P) at the instant the backtrace IPI was taken:

| Call trace:
|  _raw_spin_unlock_irqrestore+0x8/0x50 (P)
|  wake_up_process+0x18/0x24
|  process_timeout+0x14/0x20
|  call_timer_fn.isra.0+0x24/0x80
| ...

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Miroslav Benes <mbenes@suse.cz>
Reviewed-by: Puranjay Mohan <puranjay12@gmail.com>
Cc: Ard Biesheuvel <ardb@kernel.org>
Cc: Josh Poimboeuf <jpoimboe@kernel.org>
Cc: Kalesh Singh <kaleshsingh@google.com>
Cc: Madhavan T. Venkataraman <madvenka@linux.microsoft.com>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20241017092538.1859841-8-mark.rutland@arm.com


Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>