build: allow the user to specify `llvm-tblgen`

This follows the same pattern as Clang and permits the user to specify
the tablegen to use via `-DLLVM_TABLEGEN=`.  This allows for
cross-compiling LLDB for a foreign target (e.g. Windows ARM64 on Windows
X64).  The LLVM dependency for LLDB in that case must be a Windows ARM64
build which cannot cross-compile llvm-tblgen due to the way that Visual
Studio works.  Instead, permit the user to have a separate tablegen
build which can be used during the build.

llvm-svn: 366639