Linux kernel's rt_sigprocmask correctly handles an unaligned user address[1].
The original issue was for qemu-user, which seems long irrelevant.

Tested locally on an AArch64 CPU and qemu-aarch64-static.

The alignment operation actually serves another purpose: when addr resides in
the last 7 bytes of a page (unaligned), check only the current page and not the
next. Update the comment.

[1]:
kernel/signal.c `SYSCALL_DEFINE4(rt_sigprocmask`
arch/arm64/include/asm/uaccess.h:raw_copy_from_user
arch/arm64/lib/copy_template.S  "alignment handled by the hardware"

PiperOrigin-RevId: 592618320
Change-Id: Ifbd05aba42f46e36e710cca940570213036b3ce0