Port bootstrap build on OpenBSD

[am11]

Contributes to #124911.

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[am11]

These path updates in src/native/libs are compatible with dotnet/arcade#16990 and the real OpenBSD system where pkg_add icu4c openssl etc. install libs and headers.

[am11]

With dotnet/arcade#16990, #129124, #129103 and #129078, we can build runtime native src/coreclr/build-runtime.sh and with -component paltests on the OpenBSD system and get all PAL tests passing.

Also cross-build succeeds with these changes and hello world works with coreclr as well as NativeAOT. OpenBSD has similarities with Android, OSX and FreeBSD, so not much "new" here just needed to run a series of trail and error with AI to get the right balance without significant amount of new code.