powerpc64/bpf: Implement PROBE_MEM32 pseudo instructions

powerpc64/bpf: Implement PROBE_MEM32 pseudo instructions

Add support for [LDX | STX | ST], PROBE_MEM32, [B | H | W | DW]
instructions.  They are similar to PROBE_MEM instructions with the
following differences:
- PROBE_MEM32 supports store.
- PROBE_MEM32 relies on the verifier to clear upper 32-bit of the
src/dst register
- PROBE_MEM32 adds 64-bit kern_vm_start address (which is stored in _R26
in the prologue). Due to bpf_arena constructions such _R26 + reg +
off16 access is guaranteed to be within arena virtual range, so no
address check at run-time.
- PROBE_MEM32 allows STX and ST. If they fault the store is a nop. When
LDX faults the destination register is zeroed.

To support these on powerpc, we do tmp1 = _R26 + src/dst reg and then use
tmp1 as the new src/dst register. This allows us to reuse most of the
code for normal [LDX | STX | ST].

Additionally, bpf_jit_emit_probe_mem_store() is introduced to emit
instructions for storing memory values depending on the size (byte,
halfword, word, doubleword).

Stack layout is adjusted to introduce a new NVR (_R26) and to make
BPF_PPC_STACKFRAME quadword aligned (local_tmp_var is increased by
8 bytes).

Reviewed-by: Hari Bathini <hbathini@linux.ibm.com>
Tested-by: Venkat Rao Bagalkote <venkat88@linux.ibm.com>
Signed-off-by: Saket Kumar Bhaskar <skb99@linux.ibm.com>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20250904100835.1100423-2-skb99@linux.ibm.com