Currently our configure script allows linux-user targets to be built
for "unknown" host CPU architectures (which must be using the TCI
interpreter). However, code like user-exec.c has host-architecture
#ifdef ladders which in practice mean you can't build linux-user for
an arbitrary host architecture. The only two host architectures which
have code in user-exec.c but aren't recognised by configure are
HPPA and m68k.
I'd like to move to an "every supported host architecture has a
linux-user/host/$(ARCH)/hostdep.h header" model (this fixes a problem
in the code currently in master where make doesn't notice it needs to
rebuild if a host arch moves from "using generic/" to "using $(ARCH)").
Host architectures which we sort-of-but-don't support are an annoying
corner case I'd like to be able to drop entirely, by either:
(1) just dropping the support outright