Auto merge of rust-lang#102438 - andrewpollack:add-target-rustc-flags, r=Mark-Simulacrum

Adding target_rustcflags to `compiletest` TargetCfg creation

Adjustment to rust-lang#102134, ensures config returned by `rustc --target foo --print cfg` accurately reflects rustflags passed via `target_rustcflags`.

Fixes breaking change of not correctly handling `x.py test ... --test-args "--target-rustcflags -Cpanic=abort --target-rustcflags -Zpanic_abort_tests"`

cc `@djkoloski`

Author: bors