Initialize unused upper bits of Vector3 arguments and return registers for

Initialize unused upper bits of Vector3 arguments and return registers for
Linux

JIT requires the unused upper bits to be cleared but native compiler
doesn't clear the upper bits. This changes clear those bits for Reverse
PInvoke arguments and PInvoke return.

Commit migrated from https://github.com/dotnet/coreclr/commit/eb4c7a280a00b344a9bb4d35172d0a0936ec5bdd