[libc++] Adjust how we guard the inclusion of unistd.h

unistd.h isn't guaranteed to exist when the target isn't Windows, in
particular if the target is bare-metal (i.e. no operating system).
Handle this by using __has_include instead, though in
filesystem/operations.cpp we already unconditionally include it so
just remove the extra include.

Differential Revision: https://reviews.llvm.org/D79784