__vfscanf_internal: fix aliasing violation (bug 26690)

As noted in <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97264>, the cast
in the call to the read_int function is an aliasing violation.  Change the
type of local variable f to a pointer to unsigned, which allows to
eliminate most casts while only adding three new ones.

(cherry picked from commit c0e9ddf5)