Fix libs.native subset on OpenBSD

[am11]

With this patch and #125294, src/native/libs/build-native.sh -cross -os openbsd -a x64 succeeds with rootfs.

Contributes to #124911.

cc @sethjackson

[sethjackson]

Nice! @am11 see #125294 where I fix some of the HAVE_XXX things. OpenBSD has a non legacy statfs impl but CMake is being weird and not detecting it and I haven’t figured it out yet.

[am11]

@sethjackson, updated so we don't have to branch on TARGET_OPENBSD but rather rely on HAVE_xx introspection. Continues to cross-build cleanly.

[sethjackson]

This gets me through libs.native too except for pal_memory.c as I haven't figured out how to fix it in #125010 yet.
And if I pull that in to just "make it build" I get this link error:
ld.lld: error: relocation R_X86_64_PC32 cannot be used against symbol '_post_threaded'; recompile with -fPIC

[am11]

ld.lld: error: relocation R_X86_64_PC32 cannot be used against symbol '_post_threaded'; recompile with -fPIC

Could you show the surrounding context of this error? Also, have you tried recreating crossrootfs with changes in #125557 to see if it helps?

[sethjackson]

ld.lld: error: relocation R_X86_64_PC32 cannot be used against symbol '_post_threaded'; recompile with -fPIC

Could you show the surrounding context of this error? Also, have you tried recreating crossrootfs with changes in #125557 to see if it helps?

I forgot about crossrootfs changes. Rebuilding with those changes seems to have fixed this. Sorry about that and thanks for the reminder! :)