client.fibers test fails on win10 due to no static syscall numbers

It's missing "filter syscall event" output b/c those events are never
called, due to the vsyscall and the lack of elision or other scheme to get
the syscall # statically.

Start with the client.fibers test and reproduce its Windows 10 failure, focusing on the missing "filter syscall event" output. Trace why the vsyscall path does not invoke those events or expose syscall numbers statically; done means the test produces the expected filtered syscall-event output on Windows 10.

Written by the indexing model from the issue text.