build: use wxneeded on openbsd
Checklist
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commit message follows commit guidelines
Affected core subsystem(s)
build
Description of change
On OpenBSD 6.0 and greater W^X is enabled by default. All executables that violate W^X need to be marked with PT_OPENBSD_WXNEEDED. In addition to this, they must be executed from a filesystem mounted with 'wxallowed'.
More info on W^X: https://en.wikipedia.org/wiki/W%5EX