[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/syslog.cpp

@@ -4,6 +4,7 @@
 
 #include <util/format.h>
 #include <j6/syscalls.h>
+#include <j6/syslog.hh>
 
 namespace j6 {