Log Message:
lang/gcc9: add gcc-9.2.0

The GNU Compiler Collection includes front ends for C, C++, Objective-C,
Fortran, Ada, Go, and D, as well as libraries for these languages
(libstdc++,...).

This is the 2019 version, initially released in April 2019.

pkgsrc-specific changes to lang/gcc8:

The PLIST file is fixed, to guarantee that all expected files are
installed properly. In lang/gcc8 it had been autogenerated.

Only those patches have been kept that were strictly necessary to build
GCC on NetBSD-amd64. The others may be added from lang/gcc8 as necessary.

Tested by bootstrapping pkgsrc using CC=$PREFIX/gcc9/bin/gcc.