RISCV: add a few deprecated aliases for CSRs

This adds the {s,u,m}badaddr CSR aliases as well as the sptbr alias.
These are for compatibility with binutils.  Furthermore, these are used
by the RISC-V Proxy Kernel and are required to enable building the Proxy
Kernel with the LLVM IAS.

The aliases here are deprecated.  These are being introduced in order to
provide a compatibility story for the existing GNU toolchain, which
still supports the deprecated spelling in the assembler.  However, in
order to encourage the migration of existing coding, we provide warnings
indicating that the aliased CSRs are deprecated and should be replaced.

Differential Revision: https://reviews.llvm.org/D101919
Reviewed By: Craig Topper