With ARCH=sh and SUBARCH=-nofpu and ASMSUBARCH=-nofpu, it looks to
me like both src/setjmp/sh/setjmp.o and src/setjmp/sh-nofpu/setjmp.s
will get included, rather than the latter replacing the former.

> When I tried this change, I started getting duplicate symbol errors when
> linking libc.so, but with the original it seems to be working fine (this
> particular case was when ARCH=arm SUBARCH=hf ASMSUBARCH=ebhf).

I didn't mean to get rid of the existing subst, but to introduce a
second one, so that src/whatever/$(ARCH)@(ASMSUBARCH)/foo.o
excludes/replaces both src/whatever/$(ARCH)/foo.o and
src/whatever/foo.o.