platform/ptrace: workaround a kernel ptrace issue on ARM64

On ARM64, when ptrace stops on a system call, it uses the x7 register to
indicate whether the stop has been signalled from syscall entry or syscall
exit. This means that we can't get a value of this register and we can't change
it. More details are in the comment for tracehook_report_syscall in
arch/arm64/kernel/ptrace.c.

This happens only if we stop on a system call, so let's queue a signal, resume
a stub thread and catch it on a signal handling.

Fixes: #5238
PiperOrigin-RevId: 352668695

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