runtime: move doAllThreadsSyscall to os_linux.go

syscall_runtime_doAllThreadsSyscall is only used on Linux. In
preparation of a follow-up CL that will modify the function to use other
Linux-only functions, move it to os_linux.go with no changes.

For #50113.

Change-Id: I348b6130038603aa0a917be1f1debbca5a5a073f
Reviewed-on: https://go-review.googlesource.com/c/go/+/383996


Trust: Michael Pratt <mpratt@google.com>
Reviewed-by: Andrew G. Morgan <agm@google.com>
Reviewed-by: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>