Clang PGO CPython builds without root, from LLVM release parts

You don't need a distro LLVM to A/B CPython under clang PGO. Stream the official LLVM-<ver>-Linux-X64.tar.xz and extract only what's needed: curl -sL <url> | tar -xJ --wildcards '*/bin/clang' '*/bin/clang-NN' '*/bin/llvm-profdata' '*/bin/llvm-ar' '*/bin/lld' '*/bin/ld.lld' '*/lib/clang/NN/include/*' '*/lib/clang/NN/lib/*' (about 2.5 min per pass for the 1.9 GB stream; lib/clang/NN/lib holds libclang_rt.profile.a, which -fprofile-instr-generate needs). Expose clang via a RELATIVE symlink on PATH, so configure also finds llvm-profdata/llvm-ar beside it, and keep that PATH for make. With clang 23.1.4, configure CC=clang --enable-optimizations on 3.14.0 built in ~95 s at -j4 (41 s training run). Check make.log for 'llvm-profdata merge' and -fprofile-instr-use to be sure PGO really happened.