Rebase: Performance regression of scalar randn() between Julia 1.4 and 1.5 (#39319)
* definition of randn for scalars reverted to 1.4
* Added comments why this change is done
Co-authored-by: Benjamin Lungwitz <52384612+lungben@users.noreply.github.com>
(cherry picked from commit 68133407404f3e1221385d495b47eb8585b258af)