HIPStdPar: add -Wl,-rpath next to -L$(TBB_LIBDIR) in MixAndMatch std_cpu_gpu

[dannybaths]

What

HIPStdPar/CXX/MixAndMatch/std_cpu_gpu/Makefile passes the TBB directory to the
linker with -L only:

$(HIPCC) ... -o $@ -L$(TBB_LIBDIR) ${LIBS} -ltbb

-L is a link-time search path. It does not end up in the binary, so the dynamic
loader has no idea where TBB lives. This change adds the matching rpath:

$(HIPCC) ... -o $@ -L$(TBB_LIBDIR) -Wl,-rpath,$(TBB_LIBDIR) ${LIBS} -ltbb

Why

On a machine with no TBB in the default loader path, the test links without
complaint and then fails to start:

./final: error while loading shared libraries: libtbb.so.12: cannot open shared object file

The Makefile already detects TBB and already knows the directory, including the
copy ROCm ships under lib/rocprofiler-systems, which is not on the default
loader path. Nothing else is needed; the directory just has to be recorded in the
binary as well as used at link time.

Validation

MI300A / gfx942, ROCm with HIPStdPar. Focused run on upstream main (20e93223):

git clone https://github.com/amd/HPCTrainingExamples.git
cd HPCTrainingExamples/tests
export HSA_XNACK=1
export ROCM_GPU=gfx942
rm -rf build && cmake . -B build
ctest -V -R '^HIPStdPar_MixAndMatch_CpuGpu$' --test-dir build

Unpatched upstream: 1/1 FAIL. The binary records no RUNPATH; ldd reports
libtbb.so.12 => not found; CTest fails at launch with the loader error above.

With this change: 1/1 PASS. Example success marker:

RUNPATH  [<rocm>/lib/rocprofiler-systems]
StdParGpu Executor took 473 ms

AAC7 proof: current-main CTest on ROCm 10.0.0 — unpatched FAIL, patched PASS.

AAC6 no-regression: MI300A, Lmod rocm/7.14.0, Slurm job 19740 on
ppac-pl1-s24-16 — 1/1 PASS (StdParGpu Executor took 473 ms).