Update to the 20180310 snapshot of GCC 9.0.1.
Forward port r469788 from lang/gcc6 since apparently people are still using
the -mretpoline option that is specific to clang, and gcc9-devel is going
to branch into gcc9 in a rew months and that one later the default version
of GCC in use:
Filter the -mretpoline command-line option, which is understood by recent versions of clang (and used in the context of the Spectre security issues), but not GCC (which uses different options for the same) from CFLAGS and CXXFLAGS. This avoids the build of this port via bootstrap, which is the default and leverages both the system compiler (clang in most cases) plus a just built version GCC, to fail due to the latter not knowing -mretpoline.
PR: 228205
Submitted by: rozhuk.im@gmail.com