Whatever was in the frame pointer register prior to clone() will no longer be
valid in the child process, so zero it to protect FP-based unwinders. Similarly,
mark the link register as undefined to protect DWARF-based unwinders.
This is only zeroing the frame pointer(s) on Arm/Thumb because of an LLVM
assembler bug: https://github.com/llvm/llvm-project/issues/115891
The kernel does define the struct, it just doesn't use it. Yet both glibc and
musl expose it directly as their public stat struct, and std.c takes it from
std.os.linux. So just define it after all.
