[libc] Treat the locks array as a bitfield

Currently we keep an internal buffer of device memory that is used to
indicate ownership of a port. Since we only use this as a single bit we
can simply turn this into a bitfield. I did this manually rather than
having a separate type as we need very special handling of the masks
used to interact with the locks.

Reviewed By: JonChesterfield

Differential Revision: https://reviews.llvm.org/D155511