selftests/bpf: rename nocsr -> bpf_fastcall in selftests

selftests/bpf: rename nocsr -> bpf_fastcall in selftests

Attribute used by LLVM implementation of the feature had been changed
from no_caller_saved_registers to bpf_fastcall (see [1]).
This commit replaces references to nocsr by references to bpf_fastcall
to keep LLVM and selftests parts in sync.

[1] https://github.com/llvm/llvm-project/pull/105417

Acked-by: Yonghong Song <yonghong.song@linux.dev>
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20240822084112.3257995-3-eddyz87@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>