POC: Add a relocatable build flag

[zanieb]

The goal here is to construct two upstreamable patches

1. Add a --enable-relocatable configure flag to gate new behavior
2. Add linker options for relocatablity (i.e., rpath) to the CPython build behind the above flag

As a PoC, we apply these patches here for Python 3.15 and skip our post-build relocatability patchelf invocations.