qemu_x86_long: don't munge to elf32 in-place

Instead, use the QEMU_KERNEL_FILE facility to perform the
change in a new file zephyr-qemu.elf. This is what will
be sent to the emulator.

Fixes an issue where opening zephyr.elf in GDB would treat
it as a 32-bit binary instead of what it actually is,
forcing the user to override the arch setting.

Fixes: #19734

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>