Tor Browser for ppc64

ppc64 is the big-endian variant of ppc64le (tracked in #28326). All commonly-used ppc64le hardware also supports ppc64, so ppc64 has the same firmware freedom advantages that ppc64le does. ppc64 is interesting from a security perspective, because certain classes of memory-safety bugs (e.g. accessing a variable with the wrong size) cause an immediate crash or otherwise obviously-wrong behavior in BE mode, while instead manifesting as a silent security vulnerability in LE mode. Porting Tor Browser to ppc64 should be relatively simple on top of the ppc64le patches.

@richard, @boklm, and I already reached a consensus on IRC that a ppc64 port of Tor Browser would be acceptable as long as the POWER community (i.e. mainly me) does the work. Alas, I forgot to file an issue for this, so I am rectifying that error now. This issue will track my efforts on porting Tor Browser to ppc64.