[BUG] git crashes with a SIGBUS on sparc64 during pull

Thank you for filling out a Git bug report! Please answer the following questions to help us understand your issue.

What did you do before the bug happened? (Steps to reproduce your issue)

1. Compile git with clang (this doesn't seem to happen with GCC, oddly)
2. Do a `git pull` on a repository that will trigger the `unpack-objects` code

What did you expect to happen? (Expected behavior)

Pull succeeds

What happened instead? (Actual behavior)

Pull fails with this error: remote: Enumerating objects: 7, done. remote: Counting objects: 100% (5/5), done. error: unpack-objects died of signal 10 fatal: unpack-objects failed

What's different between what you expected and what actually happened?

The pull process crashes with a SIGBUS.

Anything else you want to add:

re: unpack-objects, I don't know how to specifically trigger that code, but I know that once `unpack-objects` is triggered it will reliably crash with the error above.

According to gdb, the crash happens on this line: #0 0x000001000019ca18 in cmd_unpack_objects (argc=<optimized out>, argv=0x1000063a4e0, prefix=<optimized out>, repo=<optimized out>) at builtin/unpack-objects.c:653 653 hdr->hdr_signature = htonl(PACK_SIGNATURE);

Overaligning the `buffer` declaration in the same file to try to get around possible alignment issues seems to be able to prevent the crash, but I don't know if it would be a proper fix for it.

Please review the rest of the bug report below. You can delete any lines you don't wish to share.

[System Info]
git version:
git version 2.48.1.262.g85cc9f2d1e
cpu: sparc64
built from commit: 85cc9f2d1ee4d65cb1edb00d4f56863185a53e0f
sizeof-long: 8
sizeof-size_t: 8
shell-path: /bin/sh
libcurl: 8.10.1
OpenSSL: OpenSSL 3.3.2 3 Sep 2024
zlib: 1.3.1
uname: Linux 6.6.30-sparc64-clang+ #1 SMP Sat Oct 26 21:22:10 WIB 2024 sparc64
compiler info: clang: 19.1.4
libc info: glibc: 2.40
$SHELL (typically, interactive shell): /bin/bash

[Enabled Hooks]