Update to the 20200924 snapshot of GCC 8.4.1.

This brings a large number of backports for front ends (five for aarch64,
two for arm and powerpc, nine for x86), machine independent passes (six-
teen for tree optimizers and middle end, three for RTL optimizers, and
seven around debug info), plus six for C, eight for C++, and four for
Fortran.