[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/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