MFH: r478311

emulators/rpcs3: drop GCC < 6 after r478266/r475856

USES=compiler:c++17-lang defines USE_GCC=yes but anything older than
lang/gcc7 have incomplete C++17 support. As this port is amd64-only
supporting old GCC is of little value.

https://gcc.gnu.org/projects/cxx-status.html

Approved by: ports-secteam blanket