Rollup merge of #127962 - jieyouxu:cleanup-dll-compiletest, r=fmease

Cleanup compiletest dylib name calculation

Use `std::env::consts::{DLL_PREFIX, DLL_EXTENSION}` for dylib name calculation which is more accurate for the various different platforms, and is more likely to be looked at by target maintainers.

cc ``@bzEq`` (as this impacts how compiletest handles AIX dll extensions)

Author: matthiaskrgr