[hipcc] Skip llc version probe when llc is not installed

[Samiii777]

Problem

hipconfig --full unconditionally executes <hip_clang_path>/llc --version, but llc is not shipped by any ROCm package. On a stock ROCm install the shell error leaks into hipconfig's output on every invocation:

$ hipconfig --full
...
AMD clang version 23.0.0git (...)
sh: 1: /opt/rocm/core-7.14/lib/llvm/bin/llc: not found
hip-clang-cxxflags :

Confirmed no ROCm package provides it:

$ for p in $(dpkg -l | grep amdrocm | awk '{print $2}'); do dpkg -L $p | grep -E "/bin/llc$"; done
(no output)

This is noise for interactive users and can confuse scripts that capture hipconfig's stderr. ROCm/hip#287 previously adjusted the llc path, but the call site still assumes the binary is present.

Fix

Check that llc exists before invoking it. When present, behaviour is unchanged.

Test

Built hipconfig from this branch and verified both paths:

Case	Before	After
llc absent (stock ROCm 7.14)	sh: 1: .../llc: not found	clean, no error
llc present (stub on HIP_CLANG_PATH)	probe runs	probe runs (unchanged)

Remaining --full output (HIP version, paths, compiler version, cxxflags/ldflags) is byte-identical, and --version/--path/--rocmpath/--compiler/--platform/--runtime/--hipclangpath/-C are unaffected.

Environment: ROCm 7.14.0~pre3, gfx1100 (RX 7900 XTX), Ubuntu 26.04.