Normalize Subtarget constructors to take a target triple string instead of

Normalize Subtarget constructors to take a target triple string instead of
Module*.

Also, dropped uses of TargetMachine where unnecessary. The only target which
still takes a TargetMachine& is Mips, I would appreciate it if someone would
normalize this to match other targets.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@77918 91177308-0d34-0410-b5e6-96231b3b80d8