signals: acquire loader lock during stackwalk on Win32 (#59840)
Lock the Windows loader lock in jl_with_stackwalk_lock to prevent
certain deadlocks when unwinding the stack, similar to how macOS
acquires the keymgr lock in jl_lock_profile_mach.
The loader lock prevents the loader from modifying internal data
structures while we're walking the stack, which could otherwise cause
crashes or hangs during backtrace collection.
Fixes some of #59650
🤖 Generated with Claude Code
