[LLVM][Runtimes] Add 'llvm-gpu-loader' to dependency list (#183601)

Summary:
This is used to run the unit tests in libc / libc++. It must exist in
the build directory's binary path, but without this dependnecy we may
not build it before running the runtimes build. This should ensure that
it's present, and only if we have tests enabled.

llvm/runtimes/CMakeLists.txt

@@ -524,6 +524,7 @@ if(build_runtimes)
                 llvm-symbolizer
                 llvm-xray
                 llvm-offload-binary
+                llvm-gpu-loader
                 not
                 obj2yaml
                 opt