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 91b7fbf393 ("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>

tools/include/uapi/linux/bpf.h

@@ -1970,6 +1970,8 @@ union bpf_attr {
  * 		program.
  * 	Return
  * 		The SMP id of the processor running the program.
+ * 	Attributes
+ * 		__bpf_fastcall
  *
  * long bpf_skb_store_bytes(struct sk_buff *skb, u32 offset, const void *from, u32 len, u64 flags)
  * 	Description