how do I find out what processor definition the TAU is?
Turn on the verbose option for compilation in the Arduino preferences, then copy the compile dialog and see what it defines.
For example, compiling for a genuine zero produces:
/Applications/arduino/Arduino-1.8.2.app/Contents/Java/portable/packages/arduino/tools/arm-none-eabi-gcc/4.8.3-2014q1/bin/arm-none-eabi-g++
-mcpu=cortex-m0plus -mthumb -c -g -Os -w -std=gnu++11 -ffunction-sections
-fdata-sections -fno-threadsafe-statics -nostdlib --param max-inline-insns-single=500
-fno-rtti -fno-exceptions -MMD -DF_CPU=48000000L
-DARDUINO=10802 -D[color=red]ARDUINO_SAMD_ZERO[/color] -DARDUINO_ARCH_SAMD -D__SAMD21G18A__
-DUSB_VID=0x2341 -DUSB_PID=0x804d -DUSBCON '-DUSB_MANUFACTURER="Arduino LLC"'
'-DUSB_PRODUCT="Arduino Zero"'
"-I/Applications/arduino/Arduino-1.8.2.app/Contents/Java/portable/packages/arduino/tools/CMSIS/4.5.0/CMSIS/Include/"
"-I/Applications/arduino/Arduino-1.8.2.app/Contents/Java/portable/packages/arduino/tools/CMSIS-Atmel/1.1.0/CMSIS/Device/ATMEL/"
"-I/Applications/arduino/Arduino-1.8.2.app/Contents/Java/portable/packages/arduino/hardware/samd/1.6.15/cores/arduino"
"-I/Applications/arduino/Arduino-1.8.2.app/Contents/Java/portable/packages/arduino/hardware/samd/1.6.15/variants/arduino_zero"
"/Applications/arduino/Arduino-1.8.2.app/Contents/Java/portable/packages/arduino/hardware/samd/1.6.15/cores/arduino/WMath.cpp"
-o "/var/folders/jz/5yb8f2hr8xjcpf0059bsfz4r0000gn/T/arduino_build_637127/core/WMath.cpp.o"
Where the part I've highlighted is what fastpin_arm_d21.h uses to define some pins.
Can I just copy the one from the zero and make it work for the tau?
Probably, if they are as compatible as you say... Or change the #if statement to include several symbols
#if defined(ARDUINO_SAMD_ZERO) || defined(ARDUINO_SAMD_TAU)