Since commit 23c85094fe18 ("proc/kcore: add vmcoreinfo note to /proc/
kcore") the kernel has exported the vmcoreinfo PT_NOTE on /proc/kcore
as well as /proc/vmcore.

LoongArch does not provide arch_crash_save_vmcoreinfo(), so the arch-
specific values are missing from the vmcoreinfo PT_NOTE. The generic
exporter in kernel/vmcore_info.c already provides NUMBER(VMALLOC_START),
but tools like crash cannot derive VMALLOC_END, the module and vmemmap
ranges, VA_BITS, or the KASLR offset of a LoongArch dump.

Since commit 18210a104bb9 ("LoongArch: Expand module virtual address
space to 2GB") the vmalloc and module boundaries are no longer stable
across kernel versions either, so vmcoreinfo is the only reliable source
for them.

Provide this function in a separate file that is built at the same time
as its caller in kernel/vmcore_info.c, and add a LOONGARCH64 section to
Documentation/admin-guide/kdump/vmcoreinfo.rst.

Reported-by: Haoran Jiang <jianghaoran@kylinos.cn>
Suggested-by: Haoran Jiang <jianghaoran@kylinos.cn>
Tested-by: Tao Cui <cuitao@kylinos.cn>
Co-developed-by: George Guo <guodongtai@kylinos.cn>
Signed-off-by: George Guo <guodongtai@kylinos.cn>
Signed-off-by: Yangyang Lian <lianyangyang@kylinos.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>