You compile the libretro core like this:
make
If you are compiling it for an ARM system, add "ARM=1" to the end of the command, like this:
make ARM=1
There are two additional ASM optimised CPU cores that can be used for ARM. Cyclone for 68k emulation and DRZ80 for z80. They can bring a speedup and may be of use for older cpus, but reduce compatibility. To enable them pass USE_CYCLONE=1 and/or USE_DRZ80=1 to make.
eg.
make ARM=1 USE_CYCLONE=1 USE_DRZ80=1