unix: fix gcc atomics feature check
unix: fix gcc atomics feature check

`__atomic_load_n()` and friends were added in gcc 4.7.0 but we still
have some centos6 machines with a vintage gcc 4.4.7 from 2012 in the
CI matrix.

PR-URL: https://github.com/libuv/libuv/pull/2888
Refs: https://github.com/libuv/libuv/pull/2886
Reviewed-By: Anna Henningsen <2bc1ecb410e142bce83bce6f212b41e1781536dc@addaleax.net>
Reviewed-By: Richard Lau <8193d74c06330af7778ef55510160e4e561ed6a1@uk.ibm.com>
Reviewed-By: Santiago Gimeno <3f17e29e343c4cf8809fb0970d81d6d8ee88d6b3@gmail.com>

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