Auto merge of #111472 - djkoloski:compiletest_cfg_current_target, r=compiler-errors

Get current target config from` --print=cfg`

Compiletest was switched to querying all targets using `--print=all-target-specs-json` and `--print=target-spec-json` in #108905. This unintentionally prevented codegen flags like `-Cpanic` from being reflected in the current target configuration. This change gets the current compiletest target config using `--print=cfg` like it was previously while still using the faster prints for getting information on all other targets.

Fixes #110850.

`@jyn514` might be interested in reviewing since they commented on the issue.
cc `@tmandry` since this issue is affecting Fuchsia.

Author: bors