rustc: Update linker flavor inference from filename

rustc: Update linker flavor inference from filename

This commit fixes what is believed to be a preexisting bug in the linker
flavor inference and additionally adds a new features. Previously if the
linker didn't end in `exe` the entire file name was compared to infer
the linker's flavor. This commit fixes the code to instead
unconditionally inspect `file_stem()` which is the relevant part we're
looking at to figure out what the linker flavor is.

Additionally this commit now also adds recognition of `clang` and clang
wrappers that end in `-clang` (which look like gcc wrappers). This
should allow clang-specific wrappers to get correctly inferred to the
`Gcc` linker flavor rather than the default linker flavor configured for
a target.