Cache `Process.pid`

[Feature #19443]

It's not uncommon for database client and similar network libraries
to protect themselves from Process.fork by regularly checking Process.pid

Until recently most libc would cache `getpid()` so this was a cheap
check to make.

However as of glibc version 2.25 the PID cache is removed and calls to
`getpid()` always invoke the actual system call which significantly degrades
the performance of existing applications.

The reason glibc removed the cache is that some libraries were bypassing
`fork(2)` by issuing system calls themselves, causing stale cache issues.

That isn't a concern for Ruby as bypassing MRI's primitive for forking
would render the VM unusable, so we can safely cache the PID.

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