Remove special x86 flag that disables tail call optimization.

Clang's tail call optimization for position independent code
only works when the callee was called (bound in GOT) before
the jump through GOT.
It does not work for some lazy bound Android tail callee.
We used special flag to disable all tail call optimization.

This change removes that global flag and depends on
clang compiler fix from upstream or the temporary patch in
https://android-review.googlesource.com/#/c/142241

See proposed llvm changes and pointers to llvm bugs in
https://android-review.googlesource.com/#/c/142792

Change-Id: Idb6e98d6b5f0e4c3676884b69eb3e767eba2491a