[perl #134221] open(FH, "+>>", undef) doesn't set O_APPEND

Unfortunately, this has broken VC++ builds on Windows (at least VC6 thru VC2015) - there is no O_ACCMODE here.

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via perlbug:  queue: perl5 status: pending release
https://rt.perl.org/Ticket/Display.html?id=134221

• [perl #134221] open(FH, "+>>", undef) doesn't set O_APPEND by Tony Cook via RT
• [perl #134221] open(FH, "+>>", undef) doesn't set O_APPEND by Steve Hay via RT
• [perl #134221] open(FH, "+>>", undef) doesn't set O_APPEND by Tony Cook via RT