bpf: Add helper macro bpf_addr_space_cast()

Introduce helper macro bpf_addr_space_cast() that emits:
rX = rX
instruction with off = BPF_ADDR_SPACE_CAST
and encodes dest and src address_space-s into imm32.

It's useful with older LLVM that doesn't emit this insn automatically.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/bpf/20240308010812.89848-12-alexei.starovoitov@gmail.com