GNU: Use -fvisibility on AIX when supported

Revise the logic from commit v3.7.0-rc1~173^2~2 (GNU: Do not use
-fvisibility on AIX or HP-UX, 2016-09-03) to add a version check.
The GCC 7 release notes [1] state that visibility support has been
added for AIX 7.1 and above.

[1] https://gcc.gnu.org/gcc-7/changes.html
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