Improve computation of {BC,LL}OBJS.

[jonathan]

Summary

Now that OBJS has grown an OBJS_SRCS_FILTER variable, use this variable
in the computation of BCOBJS and LLOBJS too. Also move BCOBJS and LLOBJS
computation to be next to the OBJS computation: this should both make
the parallel structure clearer and serve to remind people changing OBJS
that parallel changes are required in BCOBJS and LLOBJS.

A side effect of this change is that BCOBJS and LLOBJS will be available
even when LLVM_LINK has not been defined, but that seems like a positive
change: there's no reason we can't ask "what bitcode files would you
generate" just because we can't link those files together into a
complete bitcode representation of a binary or library.

[jonathan]

• Prefer += to = when computing {BC,LL}OBJS.

[sjg]

Ok so apart from moving the definitions, the key improvement is suppressing .bco etc. for .asm ?

[jonathan]

In D12701#263694, @sjg wrote:

Ok so apart from moving the definitions, the key improvement is suppressing .bco etc. for .asm ?

Yup, that's right.