from https://github.com/ARM-software/optimized-routines,
commit 04884bd04eac4b251da4026900010ea7d8850edc,
with minor changes to better fit into musl.

code size change: +289 bytes.
benchmark on x86_64 before, after, speedup:

-Os:
  logf rthruput:   8.40 ns/call  6.14 ns/call 1.37x
   logf latency:  31.79 ns/call 24.33 ns/call 1.31x
-O3:
  logf rthruput:   8.43 ns/call  5.58 ns/call 1.51x
   logf latency:  32.04 ns/call 20.88 ns/call 1.53x