The compiler doesn't support 32bit compatibility in an x86_64 build. You would use the compiler targeting 32bit mode, which will run well on x86_64. If you really wish to target i386, Intel calls it ia32, but I doubt you would find a use for it. You can choose to support older cpu such as sse3 and also include optimization for a newer one such as avx.
Have you considered building two dynamic libraries (one 32-bit, the other 64-bit) then have a dual architecture stub program load and call the appropriate dynamic library?
Intel Compiler 18.0 documentation clearly says you should build separate 32-bit and 64-bit binaries and combine them in a fat one