gdc bootstrap failure on alpha-linux

seen on the gcc-7 branch, and now on trunk as well, with the gdc frontend merged.

https://buildd.debian.org/status/fetch.php?pkg=gcc-8&arch=alpha&ver=8-20180130-1&stamp=1517382957&raw=0

make[5]: Entering directory '/<<PKGBUILDDIR>>/build'
rm -f stage_current
make[5]: Leaving directory '/<<PKGBUILDDIR>>/build'
Comparing stages 2 and 3
warning: gcc/cc1objplus-checksum.o differs
warning: gcc/cc1obj-checksum.o differs
Bootstrap comparison failure!
gcc/d/lexer.o differs
Makefile:24128: recipe for target 'compare' failed
make[4]: *** [compare] Error 1
make[4]: Leaving directory '/<<PKGBUILDDIR>>/build'
Makefile:24107: recipe for target 'stage3-bubble' failed
make[3]: *** [stage3-bubble] Error 2
make[3]: Leaving directory '/<<PKGBUILDDIR>>/build'
Makefile:24170: recipe for target 'bootstrap' failed
make[2]: *** [bootstrap] Error 2

the object files seem to be different ones on the branch and trunk.