pthread_yield() is non-standard and not available on Android.
shed_yield() is a POSIX standard function.

Change-Id: I093047031dd0b35a28724540aaddeb3f5ba849bc
Reviewed-on: https://swiftshader-review.googlesource.com/2682Tested-by: Nicolas Capens <capn@google.com>
Reviewed-by: Greg Hartman <ghartman@google.com>
Tested-by: Greg Hartman <ghartman@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>