[ARM] Add 'fillValidCPUArchList' to ARM targets

This is a support change for a CFE change (https://reviews.llvm.org/D42978)
that allows march and -target-cpu to list the valid targets in a note. The changes
are limited to the ARM/AArch64, since this is the only target that gets the CPU
list from LLVM.

llvm-svn: 324623