Simplify DefaultCPU in ARMTargetInfo

Simplifying the convoluted CPU handling in ARMTargetInfo.

The default base CPU on ARM is ARM7TDMI, arch ARMv4T, and
ARMTargetInfo had a different one. This wasn't visible from
Clang because the driver selects the defaults and sets the
Arch/CPU features directly, but the constructor depended
on the CPU, which was never used.

This patch corrects the mistake and greatly simplifies
how CPU is dealt with (essentially by removing the duplicated
DefaultCPU field).

Tests updated.

llvm-svn: 249699