configure: Improve check for built-in __atomic_* functions

With AC_SEARCH_LIBS() we don't succeed if the searched function is a
built-in as the check uses the wrong signature so the built-in will not
be applied (the warning issued by GCC is "conflicting types for built-in
function '...'").  So even if not required, libatomic will be linked if
it is found, which could be problematic if compiling on a separate host
and the target host does not have libatomic installed.

Also, some tests showed that it's more likely that __atomic_and_fetch()
requires linking libatomic than __atomic_load_n() does.

References #1533.

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