Fix fieldAlignment check for AtomicLong on Z

[matthewhall2]

"value" field for in AtomicLong is of type "long" and therefore should be aligned to 8-byte boundary.

checkFieldAlignmentForAtomicLong was incorrectly checking that the offset of the field is a multiple of 4 instead of a multiple of 8.

This PR fixes that.

addresses https://github.com/eclipse-openj9/openj9/issues/20235