it's trying to call a libc that does not exist. Or better: it's calling a libc that exists but it's not under the name the splunk executable expects.
this one is actually easy to fix. You need to create a symbolic link so the library ld-linux.so.3 also answer as ld-linux-armhf.so.3:
ln -s /lib/ld-linux.so.3 /lib/ld-linux-armhf.so.3
Then problem solved. Assuming the architecture of your NAS is at least ARMv6 it will work beautifully. My synology unfortunately is ARMv5 so I get an "illegal instruction" error when I run the splunk executable. Check the architecture with: