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/compilation/aot_llvm.h

@@ -408,6 +408,12 @@ typedef struct AOTCompOption {
     uint32 custom_sections_count;
 } AOTCompOption, *aot_comp_option_t;
 
+bool
+aot_compiler_init(void);
+
+void
+aot_compiler_destroy(void);
+
 AOTCompContext *
 aot_create_comp_context(AOTCompData *comp_data, aot_comp_option_t option);