ensure JSON-defined targets are consistent

[RalfJung]

We have a check_consistency check that ensures some invariants which (presumably) the rest of the compiler relies on. However, JSON targets can easily be written in a way that violates those invariants. So this PR applies the same consistency check to JSON targets that we already enforce for built-in targets.

I have converted many of the assertions in that function to new macros that show a nice error instead of a panic; if people are okay with the general approach here, I can do that for the rest of the checks as well.

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[rustbot]

These commits modify compiler targets.
(See the Target Tier Policy.)

[RalfJung]

I assume this will cause a bunch of breakage for custom target specs out there as well... the default relocation_model is "pic", but the default for dynamic_linking is false, and that's not a coherent combination (or at least, it is rejected by our existing target checks). Therefore, all targets must explicitly set one of those two fields.

[RalfJung]

Yeah, most of our tests violate this.

So either it's actually not so bad to have "relocation_model: pic, dynamic_linking: false", or we should really use different defaults...

[RalfJung]

We need some experts for linking and target specs... Cc @bjorn3 , not sure who else?

[RalfJung]

I've made it so that JSON targets may violate this particular check. Given that we also allow uefi targets to violate it, I assume this invariant is not too critical.

[bjorn3]

The combination of relocation model set to PIC and no dynamic linking is definitively fine when static_position_independent_executables is enabled. In fact static_position_independent_executables requires either PIC or PIE. When static_position_independent_executables is false, I think it would also be fine as the linker can just fill in a fixed address in the GOT, but I'm not entirely sure linkers actually support that.