Use a macro to impl FromStr for target spec data structures

[fee1-dead]

cleans up the code significantly and leaves a single source of truth for the correspondence between variants and their strings.

r? @Noratrieb

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[rustbot]

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

Some changes occurred in cfg and check-cfg configuration

cc @Urgau

[Noratrieb]

this is basically the same as ef608f1

i like my macro a bit more