bpf: __bpf_fastcall for bpf_get_smp_processor_id in uapi

bpf: __bpf_fastcall for bpf_get_smp_processor_id in uapi

Since [1] kernel supports __bpf_fastcall attribute for helper function
bpf_get_smp_processor_id(). Update uapi definition for this helper in
order to have this attribute in the generated bpf_helper_defs.h

[1] commit 91b7fbf3936f ("bpf, x86, riscv, arm: no_caller_saved_registers for bpf_get_smp_processor_id()")

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20240916091712.2929279-3-eddyz87@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>