I know IDA Pro can disassemble them to assembly code, but are there tools to decompile 64bit ARMv8 Android .so shared libraries to C?
The first tool is IDA decompiler (a plugin for IDA). As far as I know for now IDA has decompilers for x86, x64, ARM32, ARM64, PPC, PPC64, and MIPS (see here for more details, each of them costs additional money and should be purchased separately).
The second is hopper which has much weaker decompiler and claims to be able to decompile arm64.
There is also a new open source tool called Ghidra from the NSA (Direct download link is unavailable from some countries, but VPN helps). It supports decompilation for a lot of platforms (because it uses the same intermediate representation for all processors) and as opposite to IDA its processor definitions are configurable and can be updated (see the sleigh language manual).
The code generated by Ghidra may be less polished and less accurate, but you have a possibility, for example, to add new instructions when you need them - see here an example (first I found, I'm sure there are more)