bpf: Add spis_*() helpers for 4-byte stack slot bitmasks

bpf: Add spis_*() helpers for 4-byte stack slot bitmasks

Add helper functions for manipulating u64[2] bitmasks that represent
4-byte stack slot liveness. The 512-byte BPF stack is divided into
128 4-byte slots, requiring 128 bits (two u64s) to track.

These will be used by the static stack liveness analysis in the
next commit.

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20260410-patch-set-v4-3-5d4eecb343db@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>