IAR: Fix `ichecks` invocation on Windows when not using short paths

In commit c7d2a17253 (IAR: Add support for C-STAT static analysis,
2025-04-08, v4.1.0-rc1~385^2) we accidentally assumed the path to
`ichecks` never has a space.  It worked because we always constructed
the path from a Windows "short path" to the compiler tool.  Avoid
relying on that.