[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",
 

src/libraries/j6/j6/syslog.hh

@@ -0,0 +1,15 @@
+#pragma once
+/// \file j6/syslog.hh
+/// Utility function for writing messages to the kernel log
+
+// The kernel depends on libj6 for some shared code,
+// but should not include the user-specific code.
+#ifndef __j6kernel
+
+namespace j6 {
+
+void syslog(const char *fmt, ...);
+
+} // namespace j6
+
+#endif // __j6kernel

src/libraries/j6/syslog.cpp

@@ -0,0 +1,26 @@
+// The kernel depends on libj6 for some shared code,
+// but should not include the user-specific code.
+#ifndef __j6kernel
+
+#include <util/format.h>
+#include <j6/syscalls.h>
+
+namespace j6 {
+
+void
+syslog(const char *fmt, ...)
+{
+    char buffer[200];
+
+    va_list va;
+    va_start(va, fmt);
+    size_t n = util::vformat({buffer, sizeof(buffer) - 1}, fmt, va);
+    va_end(va);
+
+    buffer[n] = 0;
+    j6_log(buffer);
+}
+
+} // namespace j6
+
+#endif // __j6kernel