powerpc: add `cur_cpu_spec` symbol to vmcoreinfo

Presently, while reading a vmcore, makedumpfile uses
`cur_cpu_spec.mmu_features` to decide whether the crashed system had
RADIX MMU or not.

Currently, makedumpfile fails to get the `cur_cpu_spec` symbol (unless
a vmlinux is passed with the `-x` flag to makedumpfile), and hence
assigns offsets and shifts (such as pgd_offset_l4) incorrecly considering
MMU to be hash MMU.

Add `cur_cpu_spec` symbol and offset of `mmu_features` in the
`cpu_spec` struct, to VMCOREINFO, so that the symbol address and offset
is accessible to makedumpfile, without needing the vmlinux file

Signed-off-by: Aditya Gupta <adityag@linux.ibm.com>
Reported-by: Sachin Sant <sachinp@linux.ibm.com>
Tested-by: Sachin Sant <sachinp@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230911091409.415662-1-adityag@linux.ibm.com