gh-109054: configure checks if libatomic is needed (#109101)

Fix building the _testcapi extension on Linux AArch64 which requires
linking to libatomic when <cpython/pyatomic.h> is used: the
_Py_atomic_or_uint64() function requires libatomic
__atomic_fetch_or_8() on this platform.

The configure script now checks if linking to libatomic is needed and
generates a new LIBATOMIC variable used to build the _testcapi
extension.

Building the _testcapi extension now uses the LIBATOMIC variable in
its LDFLAGS, since Modules/_testcapi/pyatomic.c uses
<cpython/pyatomic.h>.

Co-authored-by: Erlend E. Aasland <erlend.aasland@protonmail.com>