[libc++][release] Do not force building the runtimes with -fPIC

There's a lot of history behind this, so here's a summary:

1. I stopped forcing -fPIC when building the runtimes in 30f305ef,
   before the LLVM 9 release back in 2019.

2. Someone complained that libc++.a couldn't be used in shared libraries
   built without -fPIC (http://llvm.org/PR43604) since the LLVM 9 release.
   This had been caused by my removal of -fPIC when building libc++.a in (1).

3. I suggested two ways of fixing the issue, the first being to force
   -fPIC back unconditionally (http://llvm.org/D104328), and the second
   being to specify that option explicitly when building the LLVM release
   (http://llvm.org/D104327). We converged on the first solution.

4. I landed D104328, which forced building the runtimes with -fPIC.
   This was included in the LLVM 13.0 release.

5. People complained about that and requested that we be able to
   customize this setting (basically we should have done the second
   solution).

This pa...