RFR (XS): 8074085: os::get_native_stack() could use some cleanup

[Всеволод Толстопятов]

Hi,
Could you please review and sponsor simple cleanup for
os::get_native_stack() in os_posix.cpp?

CR: *https://bugs.openjdk.java.net/browse/JDK-8074085
<https://bugs.openjdk.java.net/browse/JDK-8074085>*
Webrev: *http://cr.openjdk.java.net/~fzhinkin/vtolstopyatov/8074085/webrev.00/
<http://cr.openjdk.java.net/~fzhinkin/vtolstopyatov/8074085/webrev.00/>*
Testing: hotspot_runtime tests, except jasm and jcoder ones

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Best regards,
Tolstopyatov Vsevolod