[Sparc] Add tail call support

This patch adds tail call support to the 32-bit Sparc backend.

Two new instructions are defined, TAIL_CALL and TAIL_CALLri. They are
encoded the same as CALL and BINDri, but are marked with isReturn so
that the epilogue gets emitted. In contrast to CALL, TAIL_CALL is not
marked with isCall. This makes it possible to use the leaf function
optimization when the only call a function makes is a tail call.

TAIL_CALL modifies the return address in %o7, so for leaf functions
the value in %o7 needs to be restored after the call. For normal
functions which uses the restore instruction this is not necessary.

Reviewed By: koakuma

Differential Revision: https://reviews.llvm.org/D51206