[runtimes] Always build libc++, libc++abi and libunwind with -fPIC

[runtimes] Always build libc++, libc++abi and libunwind with -fPIC

Building the libraries with -fPIC ensures that we can link an executable
against the static libraries with -fPIE. Furthermore, there is apparently
basically no downside to building the libraries with position independent
code, since modern toolchains are sufficiently clever.

This commit enforces that we always build the runtime libraries with -fPIC.
This is another take on D104327, which instead makes the decision of whether
to build with -fPIC or not to the build script that drives the runtimes'
build.

Fixes http://llvm.org/PR43604.

Differential Revision: https://reviews.llvm.org/D104328