Imported from upstream relibc master: redox_fcntl_v0 in src/platform/redox/libredox.rs
and a thin redox_rt::sys::fcntl(fd, cmd, arg) wrapper around syscall::fcntl.
redox-scheme 0.11.2 from crates.io references redox_fcntl_v0 directly (it is
declared as an extern in libredox 0.1.18 and our local fork has it). Without
this symbol, every user daemon that uses redox-scheme 0.11.2 fails to link
with: undefined reference to redox_fcntl_v0.
This is the kind of upstream-tracking gap that the AGENTS.md fork-sync
policy exists to close. Upstream already had the function; we just had not
replayed that part of upstream history into the fork yet.
Bump the relibc fork forward in functional terms; no Cargo.toml version
change (per AGENTS.md fork-sync policy, the upstream-tag part stays
'0.2.5' and the rb-counter resets on a real upstream bump, not on
function additions).