Regarding POWER and Meltdown/Spectre
IBM has a general support document on the subject, as well as a POWER-specific blog post.
@catsay’s thread about processors immune to Spectre mentions POWER6 as possibly unaffected.
Red Hat’s page on Meltdown/Spectre curiously leaves out POWER9 big endian, but this might just be because the POWER9 chips currently available are all PowerNV, and most people run Little Endian Linux on them, and Big Endian Linux under PowerVM (PowerVM optimised chips are coming out later this year). Still, it’s a strange omission.
There are 3 known CVEs related to this issue in combination with Intel, AMD, and ARM architectures. Additional exploits for other architectures are also known to exist. These include IBM System Z, POWER8 (Big Endian and Little Endian), and POWER9 (Little Endian).
If you want more in general about Meltdown/Spectre, might I shamelessly recommenced my summary post in another thread. It has a pretty table, but the SEI/CMU CERT page is more comprehensive.