[tlbuild] Support for musl

[Karl Berry]

      i386-linux-musl
      x86_64-linux-musl

I prefer no second -, that is,
i386-linuxmusl
x86_64-linuxmusl
(Does anyone actually care about 32 bit?)

I don't have any specific reason, but on the other hand, I don't want to
go to three part names unless there is a clear need. (This is the reason
I asked Mojca to use the two-part x86_64-darwinlegacy when that started up.)

Best,
Karl