git-write-tree writes garbage on sparc64

git-write-tree writes garbage on sparc64

In the "next" branch, write_index_ext_header() writes garbage on a
64-bit big-endian machine; the written index file will be unreadable.
I noticed this on NetBSD/sparc64. Reproducible with:

$ git init-db
$ :>file
$ git-update-index --add file
$ git-write-tree
$ git-update-index
error: index uses  extension, which we do not understand
fatal: index file corrupt

Signed-off-by: Dennis Stosberg <redacted>
Signed-off-by: Junio C Hamano <redacted>