Provide linux-{image,headers}-generic virtual packages on most architectures

The Debian installer can choose a suitable kernel to install based on hardware information, but image builders can't do that and they need to know a sensible default kernel for each architecture.

I've resisted defining such a default before now, because there is no reasonable default on some architectures. But we can still provide a default for those architectures where there is:

• [alpha,sparc64] default to SMP flavour; both flavours should run on everything but this seems more useful
• [amd64] default to amd64, which should run on everything
• [arm64] default to arm64, which should run on everything
• [armhf] default to armmp, which should run on everything; non-LPAE CPUs are very common
• [hppa] default to parisc, which should run on everything
• [i386] default to 686-pae, which requires PAE; non-PAE CPUs are quite rare
• [ia64] default to itanium, which should run on everything
• [m68k,mips64r6*,ppc64*,riscv64,s390x] default to the only flavour
• [mipsr6*] default to 32-bit flavour; 64-bit systems should use the 64-bit architecture

The architectures with no default are:

• [armel,mips,mips64,mips64el,mipsel,sh4] flavours are platform-specific
• [powerpc] 32-bit flavour won't boot on 64-bit systems and vice versa

Closes: #960209