Remove extraneous eightbytes check for native structures and add tests. (#21590)

Remove extraneous eightbytes check for native structures and add tests. (#21590)

* Remove extraneous eightbytes check and add tests.

* Interger -> Integer

* Missed Helper.cs

* Handle field sizes larger than 8 bytes in AssignClassifiedEightByteTypes

* Move CoreFX test case into CoreCLR.  Fix the SystemV eightbyte classifier to correctly classify the second eightbyte when a single field crosses the eightbyte boundary (such as an in-place array of three 4-byte enums).

* Enable passing user defined structs in in-place arrays in a structure if SystemV ABI expects it.

* Correctly handle a field spanning two full eightbytes.

* Just directly assign 0 to accumulatedSizeForEightByte

* Change multi-eightbyte field handling to be a loop as per PR feedback.

* Remove extraneous whitespace.