Add port::GetProcessID() (#8693)

Add port::GetProcessID() (#8693)

Summary:
Useful in some places for object uniqueness across processes.
Currently used for generating a host-wide identifier of Cache objects
but expected to be used soon in some unique id generation code.

`int64_t` is chosen for return type because POSIX uses signed integer type,
usually `int`, for `pid_t` and Windows uses `DWORD`, which is `uint32_t`.

Future work: avoid copy-pasted declarations in port_*.h, perhaps with
port_common.h always included from port.h

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8693

Test Plan: manual for now

Reviewed By: ajkr, anand1976

Differential Revision: D30492876

Pulled By: pdillinger

fbshipit-source-id: 39fc2788623cc9f4787866bdb67a4d183dde7eef