brix@fangorn ~ $ avr-gcc --version
avr-gcc (GCC) 3.4.5 (Gentoo 3.4.5, ssp-3.4.5-1.0, pie-8.7.9)
Copyright (C) 2004 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

brix@fangorn ~ $ avr-gcc --target-help

Target specific options:
  -msize                    Output instruction sizes to the asm file
  -mshort-calls             Use rjmp/rcall (limited range) on >8K devices
  -mno-tablejump            Do not generate tablejump insns
  -mtiny-stack              Change only the low 8 bits of the stack pointer
  -mcall-prologues          Use subroutines for function prologue/epilogue
  -mno-interrupts           Change the stack pointer without disabling interrupts
  -mint8                    Assume int to be 8 bit integer
  -mmcu=                    Specify the MCU name
  -minit-stack=             Specify the initial stack address

There are undocumented target specific options as well.
AVR options:
  -mmcu=[avr-name] select microcontroller variant
                   [avr-name] can be:
                   avr1 - AT90S1200, ATtiny1x, ATtiny28
                   avr2 - AT90S2xxx, AT90S4xxx, AT90S8xxx, ATtiny22
                   avr3 - ATmega103, ATmega603
                   avr4 - ATmega83, ATmega85
                   avr5 - ATmega161, ATmega163, ATmega32, AT94K
                   or immediate microcontroller name.
  -mall-opcodes    accept all AVR opcodes, even if not supported by MCU
  -mno-skip-bug    disable warnings for skipping two-word instructions
                   (default for avr4, avr5)
  -mno-wrap        reject rjmp/rcall instructions with 8K wrap-around
                   (default for avr3, avr5)
Known MCU names:
  avr1 avr2 avr3 avr4 avr5 at90s1200 attiny10 attiny11 attiny12 attiny15
  attiny28 at90s2313 at90s2323 at90s2333 at90s2343 attiny22 attiny26
  at90s4433 at90s4414 at90s4434 at90s8515 at90s8535 at90c8534 at86rf401
  attiny13 attiny2313 attiny25 attiny45 attiny85 atmega603 atmega103
  at43usb320 at43usb355 at76c711 atmega48 atmega8 atmega83 atmega85
  atmega88 atmega8515 atmega8535 atmega16 atmega161 atmega162 atmega163
  atmega164 atmega165 atmega168 atmega169 atmega32 atmega323 atmega324
  atmega325 atmega329 atmega3250 atmega3290 atmega64 atmega644 atmega128
  atmega645 atmega6450 atmega6490 at90can128 at94k
  no emulation specific options.