Move elf32.em and elf-generic.em functions

[Nick Alcock]

On 11 Sep 2019, Alan Modra stated:

> Many ELF linker targets support multiple "emulations" and thus have
> multiple copies of elf32.em being compiled and linked into ld.  This
> patch moves much of elf32.em and elf-generic.em into files which will
> be compiled just once, resulting in a 20% decrease in ld size for
> --enable-targets=all.

As someone who just had to grok this for the first time, it was an
incredibly confusing tangle. This looks much better, no worse for ELF
than for anything else.