riscv: boot: Always make Image from vmlinux, not vmlinux.unstripped

Since commit 4b47a3aefb29 ("kbuild: Restore pattern to avoid stripping
.rela.dyn from vmlinux") vmlinux has .rel*.dyn preserved. Therefore, use
vmlinux to produce Image, not vmlinux.unstripped.

Doing so fixes booting a RELOCATABLE=y Image with kexec. The problem is
caused by this chain of events:

- Since commit 3e86e4d74c04 ("kbuild: keep .modinfo section in
vmlinux.unstripped"), vmlinux.unstripped gets a .modinfo section.
- The .modinfo section has SHF_ALLOC, so it ends up in Image, at the end
of it.
- The Image header's image_size field does not expect to include
.modinfo and does not account for it, since it should not be in Image.
- If .modinfo is large enough, the file size of Image ends up larger
than image_size, which eventually leads to it failing
sanity_check_segment_list().

Using vmlinux instead of vmlinux.unstripped means that the unexpected
.modinfo section is gone from Image, fixing the file size problem.

Cc: stable@vger.kernel.org
Fixes: 3e86e4d74c04 ("kbuild: keep .modinfo section in vmlinux.unstripped")
Signed-off-by: Vivian Wang <wangruikang@iscas.ac.cn>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Tested-by: Han Gao <gaohan@iscas.ac.cn>
Link: https://patch.msgid.link/20251230-riscv-vmlinux-not-unstripped-v1-1-15f49df880df@iscas.ac.cn
Signed-off-by: Paul Walmsley <pjw@kernel.org>