[libj6] Update how init args are passed and used

Now the init args are a linked list - this also means ld.so can use its
own plus those of the program (eg, SLP and VFS handles). __init_libj6
now adds the head of the list to its global init_args structure, and the
j6_find_init_handle function can be used to find a handle in those args
for a given proto.

This fixes situations like the logger using the wrong mailbox for the
service locator and never finding the uart driver.

src/libraries/libc/arch/amd64/crt/crt0.s

@@ -18,6 +18,8 @@ global _libc_crt0_start:function (_libc_crt0_start.end - _libc_crt0_start)
 
 _start:
 _libc_crt0_start:
+    mov rdx, [rsp]      ; grab args pointer
+
     push 0              ; Add null frame
     push 0
     mov rbp, rsp