Avoid initialize LLVM repeatedly (#1671)

Currently we initialize and destroy LLVM environment in aot_create_comp_context
and aot_destroy_comp_context, which are called in wasm_module_load/unload,
and the latter may be invoked multiple times, which leads to duplicated LLVM
initialization/destroy and may result in unexpected behaviors.

Move the LLVM init/destroy into runtime init/destroy to resolve the issue.

core/iwasm/include/aot_export.h

@@ -63,6 +63,12 @@ typedef struct AOTCompOption {
     uint32_t custom_sections_count;
 } AOTCompOption, *aot_comp_option_t;
 
+bool
+aot_compiler_init(void);
+
+void
+aot_compiler_destroy(void);
+
 aot_comp_context_t
 aot_create_comp_context(aot_comp_data_t comp_data, aot_comp_option_t option);