`std.zig.target`: Handle m68k in muslArchName().

[alexrp]

This was the only missing "proper" architecture, I think. This function also needs to be updated to handle N32 on MIPS and x32 on x86-64, but that's a problem for another day (we don't have musl-specific std.Target.Abi tags for these).

[nektro]

names listed here: https://musl.libc.org/doc/1.1.24/manual.html