Revert "linux32: add a hack to avoid redefining the type of the savefpu tag"

This reverts commit 0f6829488ef32142b9ea1c0806fb5ecfe0872c02.
Also it changes the type of md_usr_fpu_save struct mdthread member
to void *, which is what uncovered this trouble. Now the save area
is untyped, but since it is hidden behind accessors, it is not too
significant. Since apparently there are consumers affected outside
the tree, this hack is better than one from the reverted revision.

PR: 258678
Reported by: cy
Reviewed by: cy, kevans, markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D32060