powerpc64/bpf: Additional NVR handling for bpf_throw

The bpf_throw() function never returns, if it has clobbered
any callee-saved register, those will remain clobbered. The
prologue must take care of saving all callee-saved registers
in the frame of exception boundary program. Later these
additional non volatile registers R14-R25 along with other
NVRs are restored back in the epilogue of exception callback.

To achieve above objective, the frame size is determined
dynamically to accommodate additional non volatile registers
in exception boundary's frame.
For non-exception boundary program, the frame size remains
optimal. The additional instructions to save & restore r14-r25
registers are emitted only during exception boundary and
exception callback program respectively.

Signed-off-by: Abhishek Dubey <adubey@linux.ibm.com>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20260124075223.6033-7-adubey@linux.ibm.com

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