Search for libfoo.a on windows as well as foo.lib

Turns out LLVM only builds libfoo.a libraries, so we're going to need this logic
to statically link librustc
auto merge of #10785 : alexcrichton/rust/omg-i-hate-windows, r=pcwalton

Turns out LLVM only builds libfoo.a libraries, so we're going to need this logic
to statically link librustc