Add mipsel-ps2-elf target

[uyjulian]

Future support for ps2dev/ps2sdk#253

(mipsel-ps2-irx will be removed in a separate PR once the above and the next binutils update is ready)

[rickgaiser]

The goal here is to remove IRX support from the toolchain?
Instead use a standard toolchain, with a separate tool to convert the ELF file to an IRX file?

This is a great idea :). Just like how the original sony toolchain worked if I'm not mistaking, but it also allows us to use the latest gcc version without patches.

Note that after merging this PR there will be 2 IOP toolchains! This takes CPU time, and disk space, so this should be a temporary solution while we move towards the new ELF based toolchain.

Merging this PR, and looking forward to other PR's. Thanks @uyjulian !

On a side-note: I've been testing with IOP ELF files also. It seems there are some debugging sections in there that can be stripped. This stripping somehow does not work on the IRX, but it does work on the ELF. So with this change I'm hoping we can shrink the IRX files a little more in the future.