Avoid passing -L "" to rustc.

[rprichard]

Currently, target.mk passes -L "" when LLVM_STDCPP_LOCATION_$(2) is empty.

This fixes #23287.

[rust-highfive]

r? @pcwalton

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[rprichard]

I've verified that Rust compiles on FreeBSD 9 with this change. It also compiles on x86_64 Linux, with or without --enable-llvm-static-stdcpp. I'm building on 32-bit Windows, with and without --enable-llvm-static-stdcpp, and it has successfully built rustc_llvm-4e7c5e5c.dll linking with libstdc++ statically and dynamically.

[brson]

@bors: r+ rollup

Thanks!

[bors]

@bors r=brson de52403

[rprichard]

I added a compiler error for empty search paths, and it uncovered another instance of -L "" -- $$(LLVM_LIBDIR_$(2)) can also be empty. I think it comes up during cross-compilation. I'm fixing it in the makefiles. I could add a commit to this PR, but it looks like my commit is already in a rollup, so maybe I should create a new PR.