[Sparc] Add Sparc V8 support

Summary:
Adds the register class implementation for Sparc.
Adds support for DW_CFA_GNU_window_save.
Adds save and restore context functionality.

On Sparc the return address is the address of the call instruction,
so an offset needs to be added when returning to skip the call instruction
and its delay slot. If the function returns a struct it is also necessary
to skip one extra instruction.

Reviewers: jyknight, mclow.lists, mstorsjo, compnerd

Reviewed By: compnerd

Subscribers: fedor.sergeev, JDevlieghere, ldionne, libcxx-commits

Differential Revision: https://reviews.llvm.org/D55763

llvm-svn: 350705