[tlbuild] Support for musl

[Henri]

On Thu, 2018-01-25 at 22:19 +0000, Karl Berry wrote:
>       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?)

The so called target triplet is somewhat standardized.
https://wiki.osdev.org/Target_Triplet

You can obtain it from your C compiler.  With gcc, clang, and icc you can use
the command line parameter -dumpmachine.

On Ubuntu:
$ gcc -dumpmachine
x86_64-linux-gnu

On Alpine:
$ gcc -dumpmachine
x86_64-alpine-linux-musl

On Debian 32-bit:
$ gcc -dumpmachine
i686-linux-gnu

On Alpine 32-bit:
$ gcc -dumpmachine
i586-alpine-linux-musl

> 
> 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