[libj6] Add formatting j6::syslog wrapper for j6_log

To replace all the places where snprintf/j6_log are called with buffers
on the stack for most frames.

src/libraries/j6/j6.module

@@ -8,6 +8,7 @@ j6 = module("j6",
         "protocol_ids.cpp",
         "syscalls.s.cog",
         "sysconf.cpp.cog",
+        "syslog.cpp",
         "thread.cpp",
     ],
     public_headers = [
@@ -19,6 +20,7 @@ j6 = module("j6",
         "j6/protocols/service_locator.h",
         "j6/syscalls.h.cog",
         "j6/sysconf.h.cog",
+        "j6/syslog.hh",
         "j6/thread.hh",
         "j6/types.h",