PAL File for Arm Zephyr Executor Runner (#12348)

### Summary
Platform file for building Zephyr applications that use Executorch as a
module. This is a simplified version of the file used in a Zephyr
development branch
[here](https://github.com/BujSet/zephyr/blob/executorch-module-integration/samples/modules/executorch/arm/hello_world/src/arm_zephyr_pal.hpp).

In the future, it may be prudent to have a PAL file for arm bare metal
(I imagine this would require a simple splicing of he PAL functions from
the `arm_executor_runner.cpp` file). Leaving that task for a separate
PR.

### Test plan
Tested against the `arm_executor_runner.cpp` that is used in the
[Arm-EthosU
tutorial](https://docs.pytorch.org/executorch/main/tutorial-arm-ethos-u.html).
Was able to successfully build the Arm Executor Runner, and run the
simple add model.

Author: BujSet

runtime/platform/default/arm_zephyr.cpp

@@ -0,0 +1,47 @@
+#include <zephyr/kernel.h>
+#include <zephyr/sys/printk.h>
+
+void et_pal_init(void) {}
+
+ET_NORETURN void et_pal_abort(void) {
+  _exit(-1);
+}
+
+et_timestamp_t et_pal_current_ticks(void) {
+  return k_uptime_ticks();
+}
+
+et_tick_ratio_t et_pal_ticks_to_ns_multiplier(void) {
+  // Since we don't know the CPU freq for your target and just cycles in the
+  // FVP for et_pal_current_ticks() we return a conversion ratio of 1
+  return {1, 1};
+}
+
+/**
+ * Emit a log message via platform output (serial port, console, etc).
+ */
+void et_pal_emit_log_message(
+    ET_UNUSED et_timestamp_t timestamp,
+    et_pal_log_level_t level,
+    const char* filename,
+    ET_UNUSED const char* function,
+    size_t line,
+    const char* message,
+    ET_UNUSED size_t length) {
+  fprintf(
+      stderr,
+      "%c [executorch:%s:%zu %s()] %s\n",
+      level,
+      filename,
+      line,
+      function,
+      message);
+}
+
+void* et_pal_allocate(size_t size) {
+  return k_malloc(size);
+}
+
+void et_pal_free(ET_UNUSED void* ptr) {
+  k_free(ptr);
+}