build.rs: if found more than one candidate, filter on arch

If we got more then one file, only take those that contain the arch name.
For ubuntu 20.04 with host architecture x86_64 and a foreign architecture of armhf
this reduces the number of candidates to 1:

  $ find /usr/lib/python3.8/ -name '_sysconfigdata*.py' -not -lname '*'
  /usr/lib/python3.8/_sysconfigdata__x86_64-linux-gnu.py
  /usr/lib/python3.8/_sysconfigdata__arm-linux-gnueabihf.py

CHANGELOG.md: add entry for cross-sysconfigdata filter on arch

CHANGELOG.md

@@ -58,6 +58,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
   - `BaseType`, `BaseLayout`, `Layout`, `Initializer` [#1596](https://github.com/PyO3/pyo3/pull/1596)
 - Remove `__doc__` from module's `__all__`. [#1509](https://github.com/PyO3/pyo3/pull/1509)
 - Remove `PYO3_CROSS_INCLUDE_DIR` environment variable and the associated C header parsing functionality.
+- Filter sysconfigdata candidates on arch when cross-compiling. [#1626](https://github.com/PyO3/pyo3/pull/1626)
 
 ### Fixed
 - Remove FFI definition `PyCFunction_ClearFreeList` for Python 3.9 and later. [#1425](https://github.com/PyO3/pyo3/pull/1425)

pyo3-build-config/src/impl_.rs

@@ -484,6 +484,20 @@ fn search_lib_dir(path: impl AsRef<Path>, cross: &CrossCompileConfig) -> Vec<Pat
         };
         sysconfig_paths.extend(sysc);
     }
+    // If we got more than one file, only take those that contain the arch name.
+    // For ubuntu 20.04 with host architecture x86_64 and a foreign architecture of armhf
+    // this reduces the number of candidates to 1:
+    //
+    // $ find /usr/lib/python3.8/ -name '_sysconfigdata*.py' -not -lname '*'
+    //  /usr/lib/python3.8/_sysconfigdata__x86_64-linux-gnu.py
+    //  /usr/lib/python3.8/_sysconfigdata__arm-linux-gnueabihf.py
+    if sysconfig_paths.len() > 1 {
+        sysconfig_paths = sysconfig_paths
+            .iter()
+            .filter(|p| p.to_string_lossy().contains(&cross.arch))
+            .map(|p| p.clone())
+            .collect::<Vec<PathBuf>>();
+    }
     sysconfig_paths
 }