rustc_target: introduce Architecture

[tamird]

Improve type safety by using an enum rather than strings.

[rustbot]

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

Some changes occurred in compiler/rustc_attr_parsing

cc @jdonszelmann

Some changes occurred in cfg and check-cfg configuration

cc @Urgau

Some changes occurred in compiler/rustc_codegen_cranelift

cc @bjorn3

Some changes occurred in compiler/rustc_codegen_ssa

cc @WaffleLapkin

Some changes occurred in compiler/rustc_codegen_gcc

cc @antoyo, @GuillaumeGomez

[rustbot]

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[nnethercote]

This seems like a good change, but I think it's large enough that it should have an MCP, meeting the criteria of "Significant internal refactorings/changes" -- it's a simple change, but one that touches a lot of files.