Missing sync tools instructions for PowerPC arch on next

[riptl]

The PowerPC target on the next branch is missing instructions on how to run llvm-tblgen.

Running gen-tablegen-arch.sh only works with LLVM 7.0.1 and with a small patch to one of the TableGen files (replacing PPCCustomInserterPseudo with Pseudo in PPCInstrHTM.td)

[kabeor]

Solved in #1898