arm-none-eabi-objdump -d src/<file1.o> src/<file2.o> "${BuildArtifactFileBaseName}.asm"
The Post Build Steps can be found at
My Project -> Properties -> C/C++ Build -> Settings -> Build Steps
The objdump command will generate an assembly with C source listing file, just include the object files you require to be dissembled in the file list.
For example you could use this line to generate a disassembly listing of both main.c and cr_startup_lpc8xx.c
arm-none-eabi-objdump -d src/main.o src/cr_startup_lpc8xx.o > "${BuildArtifactFileBaseName}.asm"