The amdgpu C source uses a redox_log() helper to write log messages.
Previously this was likely a no-op stub. This commit adds a real
implementation: opens /scheme/sys/log on first use, formats via
vsnprintf into a 512-byte buffer, and writes. Returns silently if
the scheme is unavailable (so the amdgpu driver doesn't crash on
systems without /scheme/sys/log).
The file is still named redox_stubs.c which is a code smell per
AGENTS.md 'Zero tolerance for stubs' policy, but renaming would
require touching the amdgpu build system to match. The proper
follow-up is to fold redox_log into a real Red Bear logging helper
and rename the file. Tracked as a follow-up.
vasilito
51d650fb74