Update to GCC 9.2 release, the second in the GCC 9 series, which fixes
some 68 bugs.

This is the default version of GCC in the Ports Collection, and it just
got a bit more polish and stability.

Both files/patch-amd64-gcc-multilib-support and
files/patch-powerpc64-no-_GNU_SOURCE [1] have been integrated upstream
(and also been part of lang/gcc9-devel already), so remove them here.

PR: 239648 [1]