[LoongArch64] Fix the TestVector256() size alignment within src/tests/Interop/StructPacking/StructPacking.cs.

[LuckyXu-HF]

Fix the TestVector256() size alignment within src/tests/Interop/StructPacking/StructPacking.cs.

• Mainly fix the size and offset mismatch for the testcase: Interop.sh - global::Program.TestEntryPoint().
• Maybe we will adjust it in the future according to [LoongArch64] add Intrinsics' API for LoongArch64. #94400 .

[shushanhf]

LGTM
Thanks

[LuckyXu-HF]

This is to fix the following failed testcase: Interop/Interop/Interop.sh - global::Program.TestEntryPoint()

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14:56:56.156 Running test: global::Program.TestEntryPoint()
Unexpected Size for DefaultLayoutDefaultPacking`1[System.Runtime.Intrinsics.Vector256`1[System.Byte]].
     Expected: 64; Actual: 48
Unexpected Offset for DefaultLayoutDefaultPacking`1[System.Runtime.Intrinsics.Vector256`1[System.Byte]].Value.
     Expected: 32; Actual: 16

Unexpected Size for SequentialLayoutDefaultPacking`1[System.Runtime.Intrinsics.Vector256`1[System.Byte]].
     Expected: 64; Actual: 48
Unexpected Offset for SequentialLayoutDefaultPacking`1[System.Runtime.Intrinsics.Vector256`1[System.Byte]].Value.
     Expected: 32; Actual: 16

Unexpected Size for SequentialLayoutMaxPacking`1[System.Runtime.Intrinsics.Vector256`1[System.Byte]].
     Expected: 64; Actual: 48
Unexpected Offset for SequentialLayoutMaxPacking`1[System.Runtime.Intrinsics.Vector256`1[System.Byte]].Value.
     Expected: 32; Actual: 16

Unexpected Size for AutoLayoutDefaultPacking`1[System.Runtime.Intrinsics.Vector256`1[System.Byte]].
     Expected: 64; Actual: 48
Unexpected Offset for AutoLayoutDefaultPacking`1[System.Runtime.Intrinsics.Vector256`1[System.Byte]].Value.
     Expected: 32; Actual: 16

Unexpected Size for AutoLayoutMinPacking`1[System.Runtime.Intrinsics.Vector256`1[System.Byte]].
     Expected: 64; Actual: 48
Unexpected Offset for AutoLayoutMinPacking`1[System.Runtime.Intrinsics.Vector256`1[System.Byte]].Value.
     Expected: 32; Actual: 16

Unexpected Size for AutoLayoutMaxPacking`1[System.Runtime.Intrinsics.Vector256`1[System.Byte]].
     Expected: 64; Actual: 48
Unexpected Offset for AutoLayoutMaxPacking`1[System.Runtime.Intrinsics.Vector256`1[System.Byte]].Value.
     Expected: 32; Actual: 16

Xunit.Sdk.EqualException: Assert.Equal() Failure: Values differ
Expected: 100
Actual:   0
   at Xunit.Assert.Equal[T](T expected, T actual, IEqualityComparer`1 comparer) in /_/src/Microsoft.DotNet.XUnitAssert/src/EqualityAsserts.cs:line 168
   at Xunit.Assert.Equal[T](T expected, T actual) in /_/src/Microsoft.DotNet.XUnitAssert/src/EqualityAsserts.cs:line 90
   at Program.<<Main>$>g__TestExecutor2|25_3(StreamWriter tempLogSw, StreamWriter statsCsvSw, <>c__DisplayClass25_0&) in /home/xuliangyu/runtime/artifacts/tests/coreclr/obj/linux.loongarch64.Debug/Managed/Interop/Interop/generated/XUnitWrapperGenerator/XUnitWrapperGenerator.XUnitWrapperGenerator/FullRunner.g.cs:line 346
14:57:04.429 Failed test: global::Program.TestEntryPoint()
...
...

[LuckyXu-HF]

Hi @jkotas , could you please review this PR? Thanks.