Between 10.1A (I believe) and 10.2B there were different compiler version strings put in the r-code by the 32-bit and 64-bit compilers.
As of 11.0 the version strings are the same. But there are still some gotchas for cross-platform compatibility, e.g. taking dependencies on $DLC/tty or $DLC/gui code, the use of the {&WINDOW-SYSTEM} pre-processor, the indiscriminate use OS-COMMAND (and other OS-* commands), shared libraries and DLLs, .NET integration, etc. etc.
But if you write "pure" ABL without such dependencies, you can in theory build it once and deploy it cross-platform in 11.0+.