RFR(T): 8251846: Replace __linux which is no longer defined

[Doerr, Martin]

Hi,

we are observing lots of "error occurred during error reporting" problems on linux on PPC64 in JDK16.

Reason:
debug.cpp uses "#if defined(__linux)". It is no longer defined in our latest build environment.
Hence, fixup code for copy of uncontext is missing.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8251846

Please review this trivial fix:
http://cr.openjdk.java.net/~mdoerr/8251846_linux_ppc_context_copy_fix/webrev.00/

Best regards,
Martin