Update xtensa LLVM version to 15.x (#1741)

Fix the issue that xtensa .aot size cannot be over 256K.

build-scripts/build_llvm.py

@@ -208,7 +208,7 @@ def main():
         },
         "xtensa": {
             "repo": "https://github.com/espressif/llvm-project.git",
-            "branch": "xtensa_release_13.0.0",
+            "branch": "xtensa_release_15.x",
         },
         "default": {
             "repo": "https://github.com/llvm/llvm-project.git",

core/iwasm/aot/arch/aot_reloc_xtensa.c

@@ -259,7 +259,9 @@ apply_relocation(AOTModule *module, uint8 *target_section_addr,
             if (relative_offset < -256 * BH_KB || relative_offset > -4) {
                 set_error_buf(error_buf, error_buf_size,
                               "AOT module load failed: "
-                              "target address out of range.");
+                              "target address out of range.\n"
+                              "Try using `wamrc --size-level=0` to generate "
+                              ".literal island.");
                 return false;
             }