add OpenBSD platform-support page #92098
                
     Merged
            
            
          
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It mentions x86_64, i686, aarch64 and sparc64 which are actively maintained and used on OpenBSD (binaries provided by standard package distribution on OpenBSD).
I volontary kept
powerpc-unknown-openbsdunmentioned as it was added by @Yn0ga in #82733, and I am unaware if it is functional or not (I doubt as I added libc support only few days ago, and stdc_charsigness was wrong). @Yn0ga maybe you comment on yourpowerpc-unknown-openbsdusage ?