[build] Address symbol visibility and DSO builds

Added an `API` macro in `j6/api.h` that expands to mark the given
declaration as a default-visible symbol. Also change `format` and
`vformat` to non-template functions, and make calls to `main`, `exit`,
and the library init functions in `_start` GOT-relative.

src/libraries/j6/init.cpp

@@ -36,13 +36,13 @@ j6_find_first_handle(j6_object_type obj_type)
     return j6_handle_invalid;
 }
 
-const j6_init_args *
+const j6_init_args * API
 j6_get_init_args()
 {
     return &init_args;
 }
 
-extern "C" void
+extern "C" void API
 __init_libj6(uint64_t arg0, uint64_t arg1)
 {
     init_args.args[0] = arg0;