internal/syscall/unix: add PidFDSendSignal for Linux
CL 520266 added pidfd_send_signal linux syscall numbers to the
syscall package for the sake of a unit test.
As pidfd_send_signal will be used from the os package, let's revert the
changes to syscall package, add the pidfd_send_signal syscall numbers
and the implementation to internal/syscall/unix, and change the above
test to use it.
Updates #51246.
For #62654.
Change-Id: I862174c3c1a64baf1080792bdb3a1c1d1b417bb4
Reviewed-on: https://go-review.googlesource.com/c/go/+/528436
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LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
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