[lldb] Build the TestRosetta.py executable with system stdlib (#78370)

This is a speculative fix for TestRosetta.py which is currently failing
on Green Dragon.

TestRosetta just makes sure we can debug an x86_64 process on Apple
Silicon. However, we're failing to build the x86_64 test binary. The
linker is failing with some warnings about libc++ and libunwind being
build for arm64 while the target binary is x86_64. I'm going to try
building with the system standard libraries instead of the just-built
ones to workaround it.