[clang][ARM] Remove non-existent arm9312 CPU

I cannot find documentation on this CPU, and it
is not supported by the Arm Compiler 5 product either.

It was likely a mistake or a different name for the
"ep9312", which is an Arm based Cirrus Logic chip.

Reviewed By: peter.smith

Differential Revision: https://reviews.llvm.org/D103024