feat(esp_system): add esp_backtrace_print_all_tasks support for RISC-V

Extends esp_backtrace_print_all_tasks() to RISC-V targets. For running
tasks, registers are captured using UNW_GET_CONTEXT(); for suspended
tasks, the saved RvExcFrame on the task stack is used. Dual-core support
follows the same IPC pattern as the Xtensa implementation.

Backtrace output adapts to the configured backtrace method:
EH-frame, frame-pointer, or register dump (with a hint to enable
frame-pointer mode).

components/esp_system/include/esp_private/fp_unwind.h

@@ -1,5 +1,5 @@
 /*
- * SPDX-FileCopyrightText: 2025 Espressif Systems (Shanghai) CO LTD
+ * SPDX-FileCopyrightText: 2025-2026 Espressif Systems (Shanghai) CO LTD
  *
  * SPDX-License-Identifier: Apache-2.0
  */
@@ -7,6 +7,8 @@
 #ifndef FP_UNWIND_H
 #define FP_UNWIND_H
 
+#include <stdint.h>
+
 #ifdef __cplusplus
 extern "C" {
 #endif
@@ -20,6 +22,34 @@ extern "C" {
  */
 void esp_fp_print_backtrace(const void *frame_or);
 
+/**
+ * @brief Generate a call stack starting at the given frame pointer.
+ *
+ * @param frame[in] Frame pointer to start unrolling from.
+ * @param callers[out] Array of callers where 0 will store the most recent caller. Can be NULL.
+ * @param stacks[out] Array of callers' stacks where 0 will store the most recent caller's stack. Can be NULL.
+ * @param depth[in] Maximum number of entries to fill in the callers and stacks arrays.
+ *                  Both arrays (when provided) must be able to hold at least this many entries.
+ *
+ * @returns Number of entries filled in the array.
+ */
+uint32_t esp_fp_get_callers(uint32_t frame, void** callers, void** stacks, uint32_t depth);
+
+/**
+ * @brief Output a single backtrace step to the serial.
+ *
+ * Defined as weak so that it can be overridden by the user, e.g. to capture each
+ * backtrace step and store it or forward it elsewhere instead of printing.
+ *
+ * @warning This function is also used to print the backtrace from the panic handler.
+ *          Overriding it therefore changes the backtrace output on a panic as well,
+ *          so any override must remain safe to call from a panic/exception context.
+ *
+ * @param pc Program counter of the backtrace step.
+ * @param sp Stack pointer of the backtrace step.
+ */
+void esp_fp_generated_step(uint32_t pc, uint32_t sp);
+
 #ifdef __cplusplus
 }
 #endif