[tlbuild] Support for musl

[Karl Berry]

    the command line parameter -dumpmachine.

In terms of using this to determine the TL platform name: the
information seems sufficient, but the potential problem is requiring a
compiler to be present on the installing machine. My sense is that
nowadays many machines do not have a compiler installed.

It seems like there must be some direct way to determine if musl is
being used. ldd output? /lib/libc.so being a symlink? Something in
sysctl or /proc or /something?

Meanwhile, no problem to avoid the whole issue by installing it via
-custom-bin ... --thanks, karl.