The i386 version of as has a few machine dependent options:

--32 | --64: Select the word size, either 32 bits or 64 bits. Selecting 32-bit implies Intel i386 architecture, while 64-bit implies AMD x86-64 architecture.
These options are only available with the ELF object file format, and require that the necessary BFD support has been included (on a 32-bit platform you have to add -enable-64-bit-bfd to configure enable 64-bit usage and use x86-64 as target platform).