AMDGPU: Fix default device handling

When no device name is specified, default to kaveri
for HSA since SI is not supported and it woud fail.

Default to "tahiti" instead of "SI" since these are
effectively the same, and tahiti is an actual device.

Move default device handling to the TargetMachine
rather than the AMDGPUSubtarget. The module ISA version
is computed from the device name provided with the target
machine, so the attributes printed by the AsmPrinter were
inconsistent with those computed in the subtarget.

Also remove DevName field from subtarget since it's redundant
with getCPU() in the superclass.

llvm-svn: 258901