Optimize arch= for "pmbootstrap aportgen"

After the 5-part series of aportgen channels related adjustments are merged (#1917 (closed)), let's tackle this:

disable binutils-* building for anything but x86_64. Right now, it's failing on v20.05 branch for these combinations:

binutils-armv7 built on aarch64

binutils-aarch64 built on armv7

Nobody is cross compiling from these arches to the other, so the packages are not useful at all.

When implementing, think about other aportgen packages too - do we really need them for anything but x86_64?

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