Fixed os::chdir return value checking in launcher.cpp.

From: Vinson Lee <vlee@freedesktop.org>
Review: https://reviews.apache.org/r/13880
Fixed clang mismatched-tags errors in process.cpp.

From: Vinson Lee <vlee@freedesktop.org>
Review: https://reviews.apache.org/r/13879
Fixed clang return-type build error in os::sendfile.

From: Vinson Lee <vlee@freedesktop.org>
Review: https://reviews.apache.org/r/13840