lang/gcc10: fix build on powerpc64 with CPUTYPE=power8 or power9, also a partial fix for powerpc64le
Backport of https://gcc.gnu.org/git/?p=gcc.git;a=commitdiff;h=d00b1b023ecfc3ddc3fe952c0063dab7529d5f7a
PR: 251670
Approved by: gerald (maintainer)