bpf, x86: allow indirect jumps to r8...r15

bpf, x86: allow indirect jumps to r8...r15

Currently the emit_indirect_jump() function only accepts one of the
RAX, RCX, ..., RBP registers as the destination. Make it to accept
R8, R9, ..., R15 as well, and make callers to pass BPF registers, not
native registers. This is required to enable indirect jumps support
in eBPF.

Signed-off-by: Anton Protopopov <a.s.protopopov@gmail.com>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20251105090410.1250500-8-a.s.protopopov@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>