[Subtarget] Create a separate SubtargetSubtargetKV struct for ProcDesc to remove fields from the stack tables that aren't needed for CPUs
The description for CPUs was just the CPU name wrapped with "Select the " and " processor". We can just do that directly in the help printer instead of making a separate version in the binary for each CPU.
Also remove the Value field that isn't needed and was always 0.
Differential Revision: https://reviews.llvm.org/D58938