[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/util/util.module

@@ -10,6 +10,7 @@ module("util",
     ],
     public_headers = [
         "util/allocator.h",
+        "util/api.h",
         "util/assert.h",
         "util/basic_types.h",
         "util/bip_buffer.h",