Update to the 20200425 snapshot of GCC 9.3.1.

This brings one fix for the tree optimizers, three for gcov, four for x86
and hree for aarch64, plus two for the Fortran and C++ front ends each,
and some ten for the C++ run-time library.

Essentially forward port r533204 | gerald | 2020-04-27 from lang/gcc9

Improve upon revision 532950 by passing GCC optimization options via
MAKE_ARGS instead of trying to do this via the environment ...

which obviates the use of GCC 8 for powerpc*. [1]

PR: 245511 [1]