x86/kexec: Merge x86_32 and x86_64 code using macros from <asm/asm.h>

Merge common x86_32 and x86_64 code in crash_setup_regs()
using macros from <asm/asm.h>.

The compiled object files before and after the patch are unchanged.

Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: David Woodhouse <dwmw@amazon.co.uk>
Cc: Baoquan He <bhe@redhat.com>
Cc: Vivek Goyal <vgoyal@redhat.com>
Cc: Dave Young <dyoung@redhat.com>
Cc: Ard Biesheuvel <ardb@kernel.org>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Link: https://lore.kernel.org/r/20250306145227.55819-1-ubizjak@gmail.com