build: fix `-Wstrict-aliasing` compiler warning

Fix strict aliasing rules violation by explicitly calling `memcpy`.

libev is treated in a special way: -Wstrict-aliasing warning are ignored
there, because:
1. libev authors claim that this is a false positive and their code is
correct in respect to aliasing rules: see
* http://lists.schmorp.de/pipermail/libev/2010q1/000943.html
* http://lists.schmorp.de/pipermail/libev/2015q2/002540.html
2. it's a common practice to ignore compiler warning in libev: see
* http://pod.tst.eu/http://cvs.schmorp.de/libev/ev.pod#COMPILER_WARNINGS
* https://github.com/tarantool/tarantool/blob/dec0e0221e183fa972efa65bb0fb658112f2196f/cmake/BuildLibEV.cmake#L6-L12

Ignoring `-Wstrict-aliasing` for libev using diagnostic pragmas does not
work, apparently, due to a bug in GNU: use the `system_header` pragma
instead — ignoring all compiler warnings coming from libev is a win-win
anyways.

Needed for #7862

NO_CHANGELOG=build
NO_DOC=build
NO_TEST=build