bpf, arm64: Add JIT support for stack arguments

bpf, arm64: Add JIT support for stack arguments

Implement stack argument passing for BPF-to-BPF and kfunc calls with
more than 5 parameters on arm64, following the AAPCS64 calling
convention.

BPF R1-R5 already map to x0-x4. With BPF_REG_0 moved to x8 by the
previous commit, x5-x7 are free for arguments 6-8. Arguments 9-12
spill onto the stack at [SP+0], [SP+8], ... and the callee reads
them from [FP+16], [FP+24], ... (above the saved FP/LR pair).

BPF convention uses fixed offsets from BPF_REG_PARAMS (r11): off=-8 is
always arg 6, off=-16 arg 7, etc. The verifier invalidates all outgoing
stack arg slots after each call, so the compiler must re-store before
every call. This means x5-x7 don't need to be saved on stack.

Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20260513045158.2402494-1-yonghong.song@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>