Rollup merge of #127960 - jieyouxu:minor-rmake-cleanup, r=fmease

Cleanup dll/exe filename calculations in `run_make_support`

Use `std::env::consts` constants since now we have access to them (unlike in Makefiles!) ^^

cc `@bzEq` (this is one of the places in our test suites that tries to compute e.g. dylib extension; using `std::env::consts::DLL_EXTENSION` should correctly return `a` for AIX)

r? `@fmease` (thank you for the suggestion in rust-lang/rust#127760 (comment), this also improves correctness for the support library!)

try-job: aarch64-apple
try-job: armhf-gnu
try-job: test-various
try-job: x86_64-msvc
try-job: x86_64-gnu-llvm-18