Rollup merge of #119619 - onur-ozkan:panic-abort-mir-opt, r=oli-obk

mir-opt and custom target fixes

From rust-lang/rust#115642 (comment)

> > Could you please test the last two commits from https://github.com/onur-ozkan/rust/commits/panic-abort-mir-opt when you have the time? The first commit should resolve the error of using the nightly flag with a stable compiler, and the second one should resolve the custom target issue.

> I tested with the two commits and the errors of using nightly flag and custom target specs were not seen.
Testing was completed for the test suites like ui, run-pass-valgrind, coverage, mir-opt, codegen, assembly, incremental.

Fixes #115642

Author: matthiaskrgr