Recompiling bash with gcc-6.3 made it much less likely to happen. After recompiling my entire system with 6.3, I haven't seen it happen yet, despite doing 8 hours straight of building.
Also recompiling my linux kernel with gcc-6.3 my ~72 hour kernel crashes seem to have gone away I had to reboot for an unrelated reason, after doing this, so only have two ~144 hour uptimes for my data.
What makes me think it may be CPU rather than GCC is that the segfault seems to happen nondeterministicly in a single-threaded bash binary running the same shell script with the same arguments each time.
Somebody also reported this oddity:
Strangely enough, the machine can run mprime for days on end without any trouble. However, an average run of the Glasgow Haskell Compiler's testsuite exhibits a handful of failures (typically segmentation faults). Even stranger, if I run a few mprime threads alongside a run of GHC's testsuite, mprime will itself sometimes crash with a segmentation fault.
So you have the same mprime binary which either crashes or not depending on whether GHC tests are running at the same time.