configure: Don't pass -mfp16-format to MSVC on ARM

The check for this option does succeed - MSVC accepts the option,
but prints a warning about it being unknown and ignored, for
each compiled object file:

    cl : Command line warning D9002 : ignoring unknown option '-mfp16-format=ieee'

The configure script only attempts to add this option on ARM,
therefore this warning isn't seen by the majority of people
building with MSVC.

Making this option into a no-op probably isn't entirely right,
but on the other hand, we don't want to litter the code that
checks for support for the option with compiler specific
conditions either.

Signed-off-by: Martin Storsjö <martin@martin.st>

configure

@@ -4711,6 +4711,7 @@ msvc_common_flags(){
             -fPIC)                ;;
             -mthumb)              ;;
             -march=*)             ;;
+            -mfp16-format=*)      ;;
             -lz)                  echo zlib.lib ;;
             -lx264)               echo libx264.lib ;;
             -lstdc++)             ;;