[TargetMachine] Clean up TargetMachine::shouldAssumeDSOLocal after x86-32...

[TargetMachine] Clean up TargetMachine::shouldAssumeDSOLocal after x86-32 specific hack is moved to X86Subtarget

With my previous commit, X86Subtarget::classifyGlobalReference has learned to
use MO_NO_FLAG for 32-bit ELF -fno-pic code, the x86-32 special case in
TargetMachine::shouldAssumeDSOLocal can be removed. Since we no longer imply
dso_local for function declarations, we can drop the ppc64 special case as well.

This is NFC in terms of Clang emitted assembly.