build: fix linker warnings with clang on Windows

Clang on Windows doesn't use pthread for now, while MinGW does. Removing
`-pthread` option with MS linker fixes the following warning:

    clang: warning: argument unused during compilation: '-pthread'
        [-Wunused-command-line-argument]

Option `--no-as-needed` is meaningless for PE output. Disabling it on
Windows fixes the following warning:

    LINK : warning LNK4044: unrecognized option '/-no-as-needed'; ignored

Fixes: 98edcbb5a ("eal/windows: introduce Windows support")

Signed-off-by: Dmitry Kozlyuk <dmitry.kozliuk@gmail.com>
Tested-by: William Tu <u9012063@gmail.com>
Tested-by: Pallavi Kadam <pallavi.kadam@intel.com>
Acked-by: Pallavi Kadam <pallavi.kadam@intel.com>