libc: Add alphasort() function

Don Martin writes:

  I'm experimenting with Capstan, trying to rule out some things for
  getting a bigger application working. I have a simple program to
  recursively list directory contents, here...

    https://github.com/dmarti/mini-ls

  It works on the host, but when I build the .so, the "capstan build" step
  works, and then when I do "capstan run"...

  $ capstan run
  OSv v0.05-501-gdbdf4f7
  Failed looking up symbol alphasort

This is missing libc functionality in OSv. To fix that, add the
alphasort function from musl.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>