[3.12] gh-105102: Fix nested unions in structures when the system byteorder is the opposite (GH-105106) (GH-114204)

(cherry picked from commit 0b541f64c472976b2fee1ec9919bc7b02a798242)

Co-authored-by: Sheidan <37596668+Sh3idan@users.noreply.github.com>