Use a portable popcount so MSVC can build the LLVM backend

[dougchansan]

__builtin_popcountll is a GCC/Clang builtin with no MSVC equivalent, so an MSVC build of the LLVM backend fails outright on main:

abi_policy.cpp(15): error C3861: '__builtin_popcountll': identifier not found

Why CI does not catch it

No job builds that combination:

job	LLVM backend	compiler
build (windows-latest, Release, cl, cl)	off — so src/backend/llvm/abi_policy.cpp is never compiled	MSVC
LLVM backend (windows-latest)	on	MSYS2/UCRT64 mingw GCC, which has the builtin

MSVC plus the LLVM backend is the configuration a local Windows toolchain actually uses (vcvars64 + an LLVM release build), and it is the one combination CI never exercises.

The change

std::bitset<64>::count() is C++17, needs no feature test or #ifdef, and lowers to popcnt on every compiler that has the instruction. This was the only __builtin_ in src/.

Verification

Built and tested on three hosts, all with -DDOLRECOMP_ENABLE_LLVM=ON:

host	toolchain	before	after
Windows	MSVC 14.50 + LLVM 20.1.8	build fails	build clean, 32/32 tests pass
Linux	GCC + LLVM 20.1.2	33/33 pass	33/33 pass
macOS arm64	Apple clang + LLVM 20.1.8	3 tests fail	3 tests fail (identical set)

The three macOS failures (modern_codegen, llvm_codegen, llvm_pipeline) are pre-existing and unrelated to this change — I confirmed by reverting only this hunk on the same tree and re-running serially, which produced the identical failure set. They are also not what blocks the macOS build; that is the Mach-O section specifier issue fixed by #26.

Note for anyone reproducing the macOS numbers: run ctest -j1. Under -j10 those tests contend over shared artifacts in the build directory and report an inconsistent subset of failures between runs.

Suggested follow-up

Adding -DDOLRECOMP_ENABLE_LLVM=ON to the MSVC CI job would close the gap that let this land.