Improve dlopen() on Apple Silicon

- Introduce MAP_JIT which is zero on other platforms
- Invent __jit_begin() and __jit_end() which wrap Apple's APIs
- Runtime dispatch to sys_icache_invalidate() in __clear_cache()

libc/runtime/runtime.h

@@ -115,6 +115,8 @@ bool32 _isheap(void *);
 /* code morphing */
 void __morph_begin(void);
 void __morph_end(void);
+void __jit_begin(void);
+void __jit_end(void);
 void __clear_cache(void *, void *);
 /* portability */
 int NtGetVersion(void) pureconst;