Fix issue with threadpool and wait for multiple objects on Linux

[janvorli]

There is a method ThreadpoolMgr::ShiftWaitArray which uses memcpy to do
a move a segment of the waitPointer and waitHandle arrays one position
down, so the source and destination ranges overlap. However, it uses
memcpy, which on Linux copies items starting from the last one. So
the arrays get corrupted after the memcpy, containing multiple copies
of the last element and not containig some elements that were expected
to move.
The fix is to use memmove which should be used when the source and
destination memory regions overlap.

[janvorli]

@jkotas Can you take a look please?

[janvorli]

Fixes #2093

[jkotas]

👍

[stephentoub]

Thanks for fixing this, @janvorli! Nice job.

A quick search shows ~600 uses of memcpy in the coreclr src. If slightly different semantics between Windows and Linux could result in a bug like this being hidden until we ran on Linux, I wonder if it'd be worth auditing those call sites to proactively look for any other cases that should have been memmove?

[janvorli]

@stephentoub I have the same concern. I'll create a PS item to track it.

[janvorli]

#2101 created to track the need to audit the memcpy usages.