RFE: support ELFOSABI_LINUX, STT_GNU_IFUNC, and STB_GNU_UNIQUE in LSB 5.0

Recent versions of gcc/binutils support a new ELF ABI type, ELFOSABI_LINUX,
which supports two new features. STT_GNU_IFUNC allows a program to register a
callback for resolving symbols, and so to affect how the resolver resolves
them. STB_GNU_UNIQUE allows symbols to be declared global across the process
space even if they are loaded by dlopen() with local scope, which is important
for some use cases of dynamically loading C++ shared libs.

These new features are not supported in some current distributions, and so are
not appropriate for LSB 4.1 or earlier, but they should be added for LSB 5.0.

There's a bug history where people have been bitten by these new features; see
bug 2718, bug 2732, and bug 3625.