Build shared object libraries on Linux

[zombiezen]

As a shortcut when releasing the initial version of the zb standard library, I decided to build all libraries statically, since otherwise we would need to do something like Nix's RPATH trickery. I didn't feel like figuring that out at the time. However, this causes confusion and there's not an overly compelling reason not to support this.

Things that probably need to happen to get this to work:

• Update the compiler wrapper scripts to update RPATH, perhaps? Not exactly sure, and needs some design thonks.
• Re-build GCC with the new shared libc as appropriate. I forget whether musl can compile as shared, but this may also feed into Build glibc #28.