[release/6.0][mono][jit] Don't memset memory in increments lower than word size

[BrzVlad]

If we memset a vt containing references, then, in order for preemptive GC and concurrent GC to work, writing to reference slots must happen atomically, otherwise GC might encounter a garbage ref.

This was uncovered by crashes in CI in the System.Text.Json.Tests running on Mono(#58828)

Customer Impact
Infrequent crashes in the GC are possible on 64 bit targets where we use preemptive suspend or concurrent GC. (Desktop and mobile targets) without the fix.
Also speeds up memset by about 15% for smaller valuetypes hitting this path on 64bit. More for extra large valuetypes.

Testing
This was tested as part of our CI test suites on main for last 2 weeks without any detected failures. The changed code is hit very frequently, as part of clearing of medium sized / large valuetypes (initobj opcode).
Manual validation done prior to submission to main.

Risk
Low Risk. The change itself is very basic and it got tested over the last few weeks as part of our test suites.

Backport of #73480

[SamMonoRT]

Approved via email. Marking as servicing approved

[carlossanlop]

The single CI failure was infra related (and the leg got stuck too):

##[warning]The macOS-10.15 environment is deprecated, consider switching to macos-11(macos-latest), macos-12 instead. For more details see https://github.com/actions/virtual-environments/issues/5583
,##[error]An error occurred while provisioning resources (Error Type: Disconnect).
,##[warning]Received request to deprovision: The request was cancelled by the remote provider.

Approved and signed off. Can merge now.