Fix / suppress warnings for clang+mingw

Switch attribute error to attribute unavailable, which apparently works
since it then broke the build :-/

Suppress unused parameter warnings. They aren't that useful for windows
functions that are just #ifndef WIN32. This is marked as a global cflag,
but something is overriding that with the clang+mingw changes, and it's
not actually that hard to suppress or fix where necessary.

Bug: 69933068
Test: mmma build/tools/libhost
Change-Id: I09adda4544bc9b56452b4fe182dba3d0b701dcbb