mir-opt and custom target fixes From https://github.com/rust-lang/rust/issues/115642#issuecomment-1879589022 > > 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