Make `BINDGEN_EXTRA_CLANG_ARGS` work with `BINDGEN_EXTRA_CLANG_ARGS_<target>`

[Emilgardis]

Suppose a user has the env var BINDGEN_EXTRA_CLANG_ARGS_<target> set. They now want to add -v to clang, so they'll set BINDGEN_EXTRA_CLANG_ARGS=-v, thinking it'll work, but it wont.

I wonder if it would be possible to make BINDGEN_EXTRA_CLANG_ARGS work in tandem with BINDGEN_EXTRA_CLANG_ARGS_<target>, simply appending BINDGEN_EXTRA_CLANG_ARGS to the end.