Make tail merging the default, except on powerPC. There was no prior art

Make tail merging the default, except on powerPC.  There was no prior art
for a target-dependent default with a command-line override; this way
should be generally usable.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@37285 91177308-0d34-0410-b5e6-96231b3b80d8