Add the runfiles directory to the runtime library search paths

[EdSchouten]

When executing a C++ binary through REv2 directly (e.g., directly as a
genrule, not as a dependency of another script/executable), the input
root will only contain a _solib_arch directory inside the executable's
runfiles directory. This means that library lookups also need to
consider rpath $ORIGIN/$executable.runfiles/$workspace/_solib_$arch/.
Only looking at ../../../_solib_$arch/ is not sufficient.

This issue isn't noticeable when doing local execution, for the reason
that rtld/dyld will first do a realpath() on the executable, meaning the
lookup of shared objects takes place outside of the sandbox, inside of
bazel-out/ directly.

This change extends LibrariesToLinkCollector to duplicate all runtime
library search paths. One for bare execution, and one for execution
inside a runfiles directory.

[EdSchouten]

@oquenchil PTAL!

[oquenchil]

LGTM. Could you please verify there aren't merge conflicts? Then I will merge.

[EdSchouten]

Thanks, @oquenchil! I've just addressed the merged conflicts. Please take another look.

[oquenchil]

LGTM

[EdSchouten]

Hey @oquenchil,
Just to double check: you're not waiting for me to merge this change, right? I don't have the permissions for that.

[Wyverald]

No action items for you, @EdSchouten. Our team will merge this. Thanks!

[oquenchil]

This causes targets to fail internally when there is a , in the target name.

ld: error: cannot open SOME_NAME.runfiles/workspace_name/_solib_k8/: No such file or directory
ld: error: cannot open  SOME_NAME.runfiles/workspace_name/some_name-compiler-k8-llvm/: No such file or directory
clang: error: linker command failed with exit code 1 (use -v to see invocation)

Could you please take a look and make sure it takes that into account?

[Wyverald]

Note that this was rolled back in e898060 due to the breakage above.

[fmeum]

@oquenchil Would you have compatibility concerns if we used -Xlinker instead of -Wl? That's the most direct fix I see. At least according to https://stackoverflow.com/questions/7221141/is-there-a-difference-between-the-wl-option-and-xlinker-option-syntax-for#comment8680282_7221187, it may even be more widely supported.

[oquenchil]

On the crosstool? Can you think of any cases where that would break? I'm not familiar with the flag.

[fmeum]

Yes, that would be set on the toolchain. GCC and clang support both flags. I know that there are compilers that support -Xlinker but not -Wl. I can't rule out the possibility that there is a compiler that only supports -Wl though.

[oquenchil]

I'm not aware either, so quite possibly out of ignorance no compatibility concerns.