[BOLT][test] Use correct normalized triple

bolt/test/lit.local.cfg wants to use the system GCC installation but it
specifies a wrong triple ("linux" instead of "linux-gnu") and relies on
clangDriver's loose GCC installation detection to pick up "*-linux-gnu".

This loose behavior may not work. Use "linux-gnu" instead.

Note: neither "linux" nor "linux-gnu" detects "linux-musl" triples, so
these tests currently fail on musl based systems.

Other files changes are cosmetic.