rtos/FreeRTOS: support ARMv8-M (Cortex-M33) stacking

[anujdeshpande]

Problem

info threads / per-task backtraces are garbage when debugging FreeRTOS on ARMv8-M targets (e.g. RP2350 Cortex-M33, RP2350_ARM_NTZ port). OpenOCD's FreeRTOS support only knows the classic Cortex-M3/M4F stack layouts.

Originally reported downstream against the Flipper One MCU firmware: flipperdevices/flipperone-mcu-firmware#51

Two distinct issues:

1. Wrong stack layout. ARMv8-M ports save an extra PSPLIM + EXC_RETURN ahead of R4–R11 (and R4–R11 follow EXC_RETURN rather than preceding it), so every register offset differs from rtos_standard_cortex_m4f*. From the saved pxTopOfStack:

   0x00  PSPLIM
   0x04  EXC_RETURN (software-saved LR)
   0x08  R4 .. 0x24 R11
   (0x28 S16..S31, only when the task used the FPU - extended frame)
   <hardware frame: R0..R3, R12, LR, PC, xPSR (+ S0..S15, FPSCR)>
   

2. Wrong frame-type selection. The cm4_fpu_enabled probe (fp_feature + CPACR) does not reliably report the FPU on ARMv8-M cores, so the basic layout was applied to every task — FPU-extended-frame tasks reconstructed as pc=lr=xpsr=0, while basic-frame tasks happened to look fine.

Fix

• Add rtos_standard_cortex_m33_stacking (basic) and rtos_standard_cortex_m33_fpu_stacking (extended FPU frame) with correct offsets.
• In freertos.c, handle ARMv8-M first and pick FPU vs basic purely from bit 4 of the software-saved EXC_RETURN at offset 0x04 (which these ports always stack), independent of the unreliable FPU detection.

Testing

Tested on an RP2350 (Cortex-M33, FPU enabled) running a FreeRTOS application built with the RP2350_ARM_NTZ port, debugged over a CMSIS-DAP probe with -rtos FreeRTOS:

• Before: tasks whose saved context used the extended (FPU) frame were decoded with the basic-frame offsets and came back as pc=lr=xpsr=0 (e.g. a task whose real PC was at frame offset 0x80 was instead read from 0x40); tasks with a basic frame happened to decode correctly.
• After: every task — both basic-frame and FPU-extended-frame — reports a correct, fully symbolized backtrace via thread apply all bt.

Builds with --enable-cmsis-dap-v2.