Define STREX result codes
Add STREX_succeeded & STREX_failed to Hdr.Global.CPU.Generic32, so components don't have to define their own versions.
SyncLib is already using Strex_Succeeded & Strex_Failed, so to avoid temporarily breaking it due to introducing duplicate definitions, I've gone with STREX_success & STREX_failed (from RiscOS/Sources/Lib/RISC_OSLib!20 (merged))