zig fails to detect musl as the native C ABI on alpine linux

[andrewrk]

executing zig targets on alpine linux shows:

...
C ABIs:
  none
  gnu (native)
  gnuabin32
  gnuabi64
...

Instead it should detect that musl is the native C ABI on alpine linux.

Here are my ideas for detecting this:

• Zig should look at its own dynamic linker. This will determine which C ABI is native.
• If Zig is statically linked, then it should look at the dynamic linker of /usr/bin/env.
• If there is a problem accessing /usr/bin/env or determining its dynamic linker, Zig should conclude that the native C ABI is musl.

This only matters really if you use --library c and in this case you can work around it with -target x86_64-linux-musl.