profiles/base: mask app-crypt/gnupg[verify-sig] (and friends)

Help Portage resolve app-crypt/gnupg[verify-sig] with the app-alternatives/gpg
transition. The any-of dep in verify-sig.eclass helps a bit, but it struggles
after we made app-alternatives/gpg depend on newer gnupg to fix another problem.

Users can unmask these locally once their upgrade is done.

The workaround in 6ef49481d5057fda6d3644af4faab54ee25ba418 seemed to do
the job until dc2c7d76c21b7b6b54428d23a5c4887546f95264 when Portage's
heuristics for circular dependencies seem to run into a wall because of
app-crypt/gnupg being in @system.

A user ran into this where media-video/ffmpeg-8[verify-sig] was selected
and Portage couldn't figure out a merge plan:
```
emerge: there are no ebuilds built with USE flags to satisfy "app-crypt/gnupg[-alternatives(-)]".
!!! One of the following packages is required to complete your request:
- app-crypt/gnupg-2.4.8-r1::gentoo (Change USE: -alternatives)
(dependency required by "app-crypt/gnupg-2.4.8-r1::gentoo[verify-sig]" [ebuild])
(dependency required by "app-alternatives/gpg-0-r2::gentoo[reference]" [ebuild])
(dependency required by "media-video/ffmpeg-8.0::gentoo[verify-sig]" [ebuild])
(dependency required by "@selected" [set])
(dependency required by "@world" [argument])
```

Signed-off-by: Sam James <sam@gentoo.org>

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