[clang-repl] Tell the LLJIT the exact target triple we use.

Some systems use a different data layout. For instance, s390x the layout of
machines with vector registers is different from the ones without. In such
cases, the JIT will automatically detect the vector registers and go out of
sync.

This patch tells the JIT what is the target triple of the generated code so that
both ends are in sync.

Discussion available in https://reviews.llvm.org/D96033. Thanks to @uweigand for
helping understand the issue.

Differential revision https://reviews.llvm.org/D102756