Support emit specified custom sections into AoT file (#1207)

And add API to get the content of custom section with
section name for both wasm file and aot file.

core/iwasm/include/aot_export.h

@@ -59,6 +59,8 @@ typedef struct AOTCompOption {
     uint32_t size_level;
     uint32_t output_format;
     uint32_t bounds_checks;
+    char **custom_sections;
+    uint32_t custom_sections_count;
 } AOTCompOption, *aot_comp_option_t;
 
 aot_comp_context_t