[VE] Support VE in libunwind

Modify libunwind to support SjLj exception handling routines for VE.
In order to do that, we need to implement not only SjLj exception
handling routines but also a Registers_ve class.  This implementation
of Registers_ve is incomplete.  We will work on it later when we need
backtrace in libunwind.

Reviewed By: #libunwind, compnerd

Differential Revision: https://reviews.llvm.org/D94591