gdb/arm: Terminate frame unwinding in M-profile lockup

gdb/arm: Terminate frame unwinding in M-profile lockup

In the lockup state the PC value of the the outer frame is irreversibly
lost. The other registers are intact so LR likely contains
PC of some frame next to the outer one, but we cannot analyze
the nearest outer frame without knowing its PC
therefore we do not know SP fixup for this frame.

The frame unwinder possibly gets mad due to the wrong SP value.
To prevent problems terminate unwinding if PC contains the magic
value of the lockup state.

Example session wihtout this change,
Cortex-M33 CPU in lockup, gdb 13.0.50.20221016-git:
----------------
  (gdb) c
  Continuing.

  Program received signal SIGINT, Interrupt.
  0xeffffffe in ?? ()
  (gdb) bt
  #0  0xeffffffe in ?? ()
  #1  0x0c000a9c in HardFault_Handler ()
      at C:/dvl/stm32l5trustzone/GPIO_IOToggle_TrustZone/Secure/Src/stm32l5xx_it.c:99
  #2  0x2002ffd8 in ?? ()
  Backtrace stopped: previous frame identical to this frame (corrupt stack?)
  (gdb)
----------------
The frame #1 is at correct PC taken from LR, #2 is a total nonsense.

With the change:
----------------
  (gdb) c
  Continuing.

  Program received signal SIGINT, Interrupt.
  warning: ARM M in lockup state, stack unwinding terminated.
  <signal handler called>
  (gdb) bt
  #0  <signal handler called>
  (gdb)
----------------

There is a visible drawback of emitting a warning in a cache buildnig routine
as introduced in Torbjörn SVENSSON's
[PATCH v4] gdb/arm: Stop unwinding on error, but do not assert
The warning is printed just once and not repeated on each backtrace command.

Signed-off-by: Tomas Vanek <vanekt@fbl.cz>