Do not do tail calls to external symbols. If the

Do not do tail calls to external symbols.  If the
branch turns out to be ARM-to-Thumb or vice versa
the linker cannot resolve this.  8120438.

If this optimization is going to be useful we probably
need a compiler flag "assume callees are same architecture"
or something like that.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@106662 91177308-0d34-0410-b5e6-96231b3b80d8