The AT90S8515 is an "old-style" (one of the very first) AVR microcontrollers, and it does not have the SPM instruction or the ability to self-program its own flash memory, which is necessary for a bootloader-based environment like Arduino to operate properly. Basically, you can't use an 90S8515 without a specific "device programmer" module, and one of those is comparable in price to a full Arduino (even the cheaper versions will likely cost as much as an Arduino clone.)
There is a newer AVR that is similar to the 90s8515 (ATmega8515), but except for pins it has less capabilities than even the older Arduino chips (like the ATmega8) Arduino variants like Sanguino (http://sanguino.cc) that use a 40-pin chip seem to have settled on the ATmega644.