[TargetParser] Generate the defs for RISCV CPUs using llvm-tblgen.

Rework the change to prevent build failures. NFCI.

The failing code was submitted as
cf7a8305 and reverted via
8bd65e53.

The rework in this new commit prevents failures like the following:

FAILED: tools/clang/lib/Basic/CMakeFiles/obj.clangBasic.dir/Targets/RISCV.cpp.o
/usr/bin/c++  [bunch of non interesting stuff]  -c <path-to>/llvm-project/clang/lib/Basic/Targets/RISCV.cpp
In file included from <path-to>/llvm-project/clang/lib/Basic/Targets/RISCV.cpp:19:
<path-to>/llvm-project/llvm/include/llvm/TargetParser/RISCVTargetParser.h:29:10: fatal error: llvm/TargetParser/RISCVTargetParserDef.inc: No such file or directory
  29 | #include "llvm/TargetParser/RISCVTargetParserDef.inc"
     |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

These failures happen because the library LLVMTargetParser depends on
RISCVTargetParserTableGen, which is a tablegen target that generates
the list of CPUs in
llvm/TargetParser/RISCVTargetParserDef.inc. This *.inc file is
included by the public header file
llvm/TargetParser/RISCVTargetParser.h.

The header file llvm/TargetParser/RISCVTargetParser.h is also used in
components (clangDriver and clangBasic) that link into
LLVMTargetParser, but on some configurations such components might end
up being built before TargetParser is ready.

The fix is to make sure that clangDriver and clangBasic depend on the
tablegen target RISCVTargetParserTableGen, which generates the .inc
file whether or not LLVMTargetParser is ready.

WRT the original patch at https://reviews.llvm.org/D137517, this
commit is just adding RISCVTargetParserTableGen in the DEPENDS list of
clangDriver and clangBasic.