windows: Copy libwinpthread-1.dll into libdir bin

windows: Copy libwinpthread-1.dll into libdir bin

Recently we switched from the win32 MinGW toolchain to the pthreads-based
toolchain. We ship `gcc.exe` from this toolchain with the `rust-mingw` package
in the standard distribution but the pthreads version of `gcc.exe` depends on
`libwinpthread-1.dll`. While we're shipping this DLL for the compiler to depend
on we're not shipping it for gcc. As a workaround just copy the dll to gcc.exe
location and don't attempt to share for now.

cc https://github.com/rust-lang/rust/issues/31840#issuecomment-297478538