Remove redundant pinning over static fields/RVA

[EgorBo]

I noticed that sometimes developers make assumptions that it's fine to use Unsafe.AsPointer over RVA fields since they're not movable, which is fairly unreliable thing to do (e.g. Unloadable ALCs). So let's just optimize the pinning overhead in JIT when we know it's safe to do so:

// Case 0: RVA fields
void Test0()
{
    ReadOnlySpan<int> rva = [1,2,3,4,5];
    fixed (int* p = rva)
        Consume(p);
}

// Case 1: Primitive static fields (unmanaged memory)
static int _primitiveFld;
void Test1()
{
    fixed (int* p = &_primitiveFld)
        Consume(p);
}


// Case 2: Struct static fields (boxed and stored on the NonGC heap)
static MyStruct _structFld;
void Test2()
{
    fixed (int* p = &_structFld.Y)
        Consume(p);
}

Codegen diff: https://www.diffchecker.com/LgIALnAn/

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[EgorBo]

@MihuBot

[jkotas]

HasGCStaticBase condition should not be needed for NativeAOT.

See dotnet/runtimelab#870