BUILD: sym: Get it to build and work on various OSes

BUILD: sym: Get it to build and work on various OSes

Define NT_GNU_BUILD_ID and STT_GNU_IFUNC, as those are GNU extensions,
and not every OS defines it.
Define ElfW() if it is not provided by the OS already.
Make sure sections are actually readable, as sometimes they are not on
OpenBSD.
The NetBSD linker does not use an empty name for the executable, so
don't recognize it that way. However, every OS will provide the
executable first, so rely on that instead.

With those changes, this builds and works on NetBSD, OpenBSD and
Solaris/Illumos.