libthr: Add a stop indicator to thread_start to satisfy unwinders

[dchagin]

Summary

The right unwinding stop indicator should be CFI-undefined PC.
https://dwarfstd.org/doc/Dwarf3.pdf - page 118:

• If a Return Address register is defined in the virtual unwind table,

and its rule is undefined (for example, by DW_CFA_undefined), then
there is no return address and no call address, and the virtual
unwind of stack activations is complete.

[kib]

This is not guaranteed to work at all. Compilers do not provide any guarantee about specific location for the emition of inline asm.