libc: add qsort_r()

qsort_r() patch for musl is available but haven't merged upstream musl:
https://github.com/esmil/musl/commit/194f9cf93da8ae62491b7386edf481ea8565ae4e
So let's add it on libc/.
This is required for Ruby 2.2.0 and Publify.

Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Takuya ASADA <syuu@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>