rustllvm: Only initialize command-line arguments once

In my WIP on rustpkg, I was calling driver code that calls
LLVMRustWriteOutputFile more than once. This was making LLVM
unhappy, since that function has code that initializes the
command-line options for LLVM, and I guess you can't do that more
than once. So, check if they've already been initialized.
auto merge of #5755 : catamorphism/rust/rustllvm-cmdline, r=brson

r? @brson In my WIP on rustpkg, I was calling driver code that calls
LLVMRustWriteOutputFile more than once. This was making LLVM
unhappy, since that function has code that initializes the
command-line options for LLVM, and I guess you can't do that more
than once. So, check if they've already been initialized.