I tried something slightly different: I asked an x86 Debian machine at work to cross-compile the gcc package for powerpc. It didn't have any crosstools installed, so the compilation failed, but it did apply the patches to the source tree so that I could copy it over to my PPC machine.
I then ran configure with the same operations as the Debian build (except different prefix and disabling all languages except C and C++) and then make, but yet again GCC fails at building itself. Crash during stage2 again:
/biggus/gcc-6-debian/build/./prev-gcc/xg++ -B/biggus/gcc-6-debian/build/./prev-gcc/ -B/opt/gcc6/powerpc-linux-gnu/bin/ -nostdinc++ -B/biggus/gcc-6-debian/build/prev-powerpc-linux-gnu/libatomic/.libs -B/biggus/gcc-6-debian/build/prev-powerpc-linux-gnu/libstdc++-v3/src/.libs -B/biggus/gcc-6-debian/build/prev-powerpc-linux-gnu/libstdc++-v3/libsupc++/.libs -I/biggus/gcc-6-debian/build/prev-powerpc-linux-gnu/libstdc++-v3/include/powerpc-linux-gnu -I/biggus/gcc-6-debian/build/prev-powerpc-linux-gnu/libstdc++-v3/include -I/biggus/gcc-6-debian/src/libstdc++-v3/libsupc++ -L/biggus/gcc-6-debian/build/prev-powerpc-linux-gnu/libatomic/.libs -L/biggus/gcc-6-debian/build/prev-powerpc-linux-gnu/libstdc++-v3/src/.libs -L/biggus/gcc-6-debian/build/prev-powerpc-linux-gnu/libstdc++-v3/libsupc++/.libs -fno-PIE -c -DIN_GCC_FRONTEND -g -O2 -gtoggle -DIN_GCC -fno-exceptions -fno-rtti -fasynchronous-unwind-tables -W -Wall -Wno-narrowing -Wwrite-strings -Wcast-qual -Wmissing-format-attribute -Woverloaded-virtual -pedantic -Wno-long-long -Wno-variadic-macros -Wno-overlength-strings -DHAVE_CONFIG_H -I. -Icp -I/biggus/gcc-6-debian/build/../src/gcc -I/biggus/gcc-6-debian/build/../src/gcc/cp -I/biggus/gcc-6-debian/build/../src/gcc/../include -I/biggus/gcc-6-debian/build/../src/gcc/../libcpp/include -I/biggus/gcc-6-debian/build/../src/gcc/../libdecnumber -I/biggus/gcc-6-debian/build/../src/gcc/../libdecnumber/dpd -I../libdecnumber -I/biggus/gcc-6-debian/build/../src/gcc/../libbacktrace -o cp/cp-objcp-common.o -MT cp/cp-objcp-common.o -MMD -MP -MF cp/.deps/cp-objcp-common.TPo /biggus/gcc-6-debian/build/../src/gcc/cp/cp-objcp-common.c /biggus/gcc-6-debian/build/../src/gcc/cp/cp-objcp-common.c: In function ¡void init_shadowed_var_for_decl()¢: /biggus/gcc-6-debian/build/../src/gcc/cp/cp-objcp-common.c:202:1: internal compiler error: Segmentation fault init_shadowed_var_for_decl (void) ^~~~~~~~~~~~~~~~~~~~~~~~~~ 0x10c39317 crash_signal /biggus/gcc-6-debian/build/../src/gcc/toplev.c:333 Please submit a full bug report, with preprocessed source if appropriate. Please include the complete backtrace with any bug report. See file:///usr/share/doc/gcc-6/README.Bugs for instructions. Makefile:1088: recipe for target 'cp/cp-objcp-common.o' failed make[3]: *** [cp/cp-objcp-common.o] Error 1 make[3]: *** Waiting for unfinished jobs....