In bug 1839743, we made the build system prefer packed relative
relocations to elfhack when both the system libc and linker support
them. Unfortunately, while that covers most of the benefits from
elfhack, it doesn't cover bug 651892.

To cover it, we make every C++ executable contain its own copy of
the symbol, so that all relocations related to it become relative.

And because this is actually (slightly) beneficial on macos, and because
it's also an advantage to have our own abort called rather than the
system's, we apply the same to all platforms.