Revert "lib{c,sys}: normalize export of openat, setcontext, and swapcontext"

I put the symbols in the wrong file (should have been
lib/libc/sys/Symbol.map), added a duplicate pdfork entry due to a botch
rebase, and there seems to be a issue with gcc13/binutils not exposing
the symbols so revert the whole thing while I debug.

This reverts commit ee632fb9eb4a060a4087a7f5425bfe36e65cda61.