android-x86 target: Add -latomic

Fixes https://github.com/openssl/openssl/issues/14083 again after being
broken by https://github.com/openssl/openssl/pull/15640.

CLA: trivial

Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/17833)

Configurations/15-android.conf

@@ -261,6 +261,7 @@ my %targets = (
         bn_ops           => add("RC4_INT"),
         asm_arch         => 'x86',
         perlasm_scheme   => "android",
+        ex_libs          => add(threads("-latomic")),
     },
     "android-x86_64" => {
         inherit_from     => [ "android" ],