[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/libc/arch/amd64/crt/crt0.s

@@ -12,17 +12,17 @@ _libc_crt0_start:
     push 0
     mov rbp, rsp
 
-    call __init_libj6
+    call __init_libj6 wrt ..got
     mov rbx, rax
 
-    call __init_libc
+    call __init_libc wrt ..got
 
     mov rdi, 0
     mov rsi, rsp
     mov rdx, 0    ; TODO: actually parse stack for argc, argv, envp
     mov rcx, rbx
-    call main
+    call main wrt ..got
 
     mov rdi, rax
-    call exit
+    call exit wrt ..got
 .end: