Build import libs as .dll.a instead of .lib

This is MinGW convention.

Signed-off-by: Timothy Gu <timothygu99@gmail.com>
.gitignore: add some more entries concerned with kernel

Signed-off-by: Timothy Gu <timothygu99@gmail.com>
Merge pull request #399 from TimothyGu/upstr

Build import libs as .dll.a instead of .lib