This has been clearly lacking while making the previous commit
but the implementation isn't that clear so let it be a separate
step.

The problem requiring the change in subsequent processors
is that these relied upon "@arch" as a flag to be inspected,
and "pkg@!arch1,arch2" on arch2 needs to take out *all* of that
fragment *including* arch1 mention as well.

Part of the cause is difference in handling: "positive" multi-match
would explode its "client" line into multiple lines to filter down
the pipeline, while "negative" multi-match *has* to keep that line
on a similarly single line (otherwise we'd end up with N-1 of those
slipping past the filter for particular architecture thus defeating
the whole purpose of "negative" matching semantics):

$ echo 'pkg@!E2K,mipsel,riscv64' |
  sed -r  ':loop; s/^((([^@]+@!)[^,]+)+),([a-zA-Z0-9_]+)/\1@!\4/; t loop'
pkg@!E2K@!mipsel@!riscv64

I've tried my best to test this specific change but it still might
introduce a regression in some corner case; feel free to report;
looks like there's a space for improvement in m-p's automated
tests department as well.

So now we can do:

  pkg@!ARCHES1,ARCHES2,arch3,arch4

and have pkg excluded on arches mentioned; the previous approach
could only offer explicit whitelists (not that it was entirely
wrong but then again, we have both ExclusiveArch and ExcludeArch
rpmtags in our spec files).

This has been inspired by a few commits that cared
for package availability reasons on a particular
architecture; the problem at hand is that pkglists
might need to include groups of packages that are
(un)available on groups of arches, and tackling that
with plain pkg@arch just results in combinatorial
explosion of that matrix.

Arches are handled one-by-one with a few hardcoded
macro substitutions.

Exploding a "pkg@arch1,arch2" string into a set of:

pkg@arch1
pkg@arch2

with subsequent archdep pruning would do the trick;
so here's another sed oneliner that does just that:

$ echo 'pkg@X86,ARM,ppc64le' |
  sed -r ':loop; s/^((([^@]+@)[^,]+)+),([^,]+)/\1\n\3\4/; t loop'
pkg@X86
pkg@ARM
pkg@ppc64le

See-also: 9601a9e7
See-also: 5581dc91
See-also: http://stackoverflow.com/a/55781741/561921

Those willing to reuse archdep-filter elsewhere
will likely have to figure it out from Makefiles
anyway, so probably not worth overdocumenting...

doc/archdep.txt was the reason to look closer,
and some other files needed a bit of facelift
by now too (would be better to do that in sync).

See commit 2d257a42
for details, this just brings those to pkg.in/profiles/
(along with the generic archdep support, of course).

The former ("proper 32-bit x86 package form") has been suggested
by zerg@ quite some time ago but the desired interface wasn't clear
at the moment IIRC; a quiet morning helped me realize that

  ICAClient-preinstall@IA32

is rather more readable than

  ICAClient-preinstall@i586 i586-ICAClient-preinstall@x86_64

so here's the (trivial) implementation; and I actually needed
the latter, @X86 ("any-x86") to mark x86-only packages so

  xorg-drv-intel@X86

is now equivalent to

  xorg-drv-intel@i586 xorg-drv-intel@x86_64

Build process would choke on "unescaped" stuff otherwise,
and these blocks should be blockquotes in the first place.

This is an initial implementation of architecture dependent
contents handling for package lists more or less in the vein
of mkimage-profiles-desktop's one *but* using suffix part to
filter words in or out *not* prefix part to replace it with
a comment marker (thus filtering out lines).

The syntax should be pretty obvious:

  a b@i586 c@x86_64

will get "a b" given ARCH=i586 and "a c" given ARCH=x86_64;
please see doc/archdep.txt for a more elaborate description
and a conversion script.