libretiny: derive nm from CC path when $NM is unset

esphome/components/libretiny/patch_linker.py.script

@@ -114,7 +114,11 @@ def _post_link(target, source, env):
     elf = env.subst("$BUILD_DIR/${PROGNAME}.elf")
     if not os.path.isfile(elf):
         return
-    nm = env.subst("$NM") or env.subst("${TOOLCHAIN_PREFIX}nm")
+    # $NM may be unset; derive from $CC (e.g. arm-none-eabi-gcc -> arm-none-eabi-nm).
+    nm = env.subst("$NM")
+    if not nm:
+        cc = env.subst("$CC")
+        nm = cc.replace("-gcc", "-nm") if cc.endswith("-gcc") else "nm"
     try:
         out = subprocess.check_output(
             [nm, "--defined-only", "--demangle", elf], text=True