Restore full PGO on the arm64-macOS standalone binary
Summary: Restore full PGO on the arm64-macOS standalone binary (v0.6.0b4).
The arm64-macOS secantusd-rs binary's on-target profile-guided optimization is
working again, after a run of macos-14-runner-specific failures. Two quirks (both
Linux-immune) are now handled: the PGO instrumented build drops mimalloc (whose
instrumented allocator internals segfault at startup), and — the crux — the
instrumented binary writes its own profile on shutdown, because the profiling
runtime on that runner never wires up its LLVM_PROFILE_FILE at-exit write (a
clean --version exit produced no .profraw at all). A CI diagnostic pinned
that down; the binary now calls __llvm_profile_set_filename +
__llvm_profile_write_file to a known path (behind the instrumented-only
pgo-instrument feature), yielding a valid, mergeable profile (23.9k functions).
Both binary targets ship with full two-stage PGO again.