Update to the 20200502 snapshot of GCC 10.0.1, which has branched for
the release of GCC 10.1 (and the GCC 10 release series) now.

Forward port r517843 | gerald | 2019-11-17 from lang/gcc9-devel since
this issue has not been addressed upstream or in our system compiler
yet. [1]

clang on rs6000/powerpc* unfortunately poisons user namespace by default
(without any special options or include files being required).

Until that changes (or GCC changes) we need to avoid using vec_step as a
variable name.

PR: 239266, 245483