bpf, arm64: Map BPF_REG_0 to x8 instead of x7

Move the BPF return value register from x7 to x8, freeing x7 for use
as an argument register. AAPCS64 designates x8 as the indirect result
location register; it is caller-saved and not used for argument
passing, making it a suitable home for BPF_REG_0.

This is a prerequisite for stack argument support, which needs x5-x7
to pass arguments 6-8 to native kfuncs following the AAPCS64 calling
convention.

Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20260513045153.2402197-1-yonghong.song@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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