Backport of #95530 to release/8.0-staging

/cc @EgorBo

Customer Impact

When jit needed to zero a struct with gc references it used to emit a call to an internal helper JIT_MemSet that didn't have atomicity guarantees and could lead to torn values if GC happened during that memset. This issue was present from .NET core 3.

Example:

void Test(ref LargeStructWithGC s)
{
    s = default; // GC may stop during this zeroing and read `a` field which might be half-zeroed.
}

struct LargeStructWithGC
{
    long b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, r, s, t, u, v, w, z;
    string a;
}

The PR fixes that issue by inlining memset so we can 100% guarantee atomicity by using correct instructions.
The original issue was found via internal escalation (service crashing 3x per day).

Testing

Local testing + stress/outerloop jobs

Risk

Low. It is not a common case. It happens for a big struct with GC types. Even our SPMI has very few of these cases.