Replace `$` with `$@` in Makefile

I was trying to build this project but it did not work because the commands were gcc ... -o $ in the Makefile, where $ probably means "output file", but on my computer, make only thought it was a dollar sign, not anything special. After replacing all occurrences of lone $ with $@, I could build the project without any errors with GCC.

I wonder if the reason this works is that on Linux, make sees $ as $@. Maybe it does not on my computer because I am using mingw64 msys2 to build posix-uefi.