memcpy/memset helpers on CoreCLR may not be suspension-friendly

[VSadov]

This came in discussions for another fix:
#98547 (comment)

We generally try avoiding unbounded operations that may happen in coop mode while running native code as they may result in arbitrary long pauses when nothing runs except the said operation.
(the operation starves GC and GC starves the rest of the app threads)

This scenario should be looked at and if needed either move to managed code, or make it happen piece-wise interleaved with suspension opportunities.