Package oslogabi builds the argument buffers of the unified logging
system's ABI, shared by the os_log (x/oslog) and os_signpost (x/signpost)
emitters. The buffer layout is produced by clang's __builtin_os_log_format
in C; the oracle test validates this encoder against clang itself.
Encode builds the os_log argument buffer for format and args. It returns
the buffer and a slice of values that must be kept alive for the duration
of the emitting call (the C strings that buffer pointers refer into).