configure: Don't pass -mthumb or -march= to MSVC

The modern MSVC for ARM always builds for thumb, and it can't be
disabled.

Also just use the default arch instead of trying to map the -march
parameter to MSVC's -arch parameter (which only takes the values
ARMv7VE and VFPv4).
Signed-off-by: Martin Storsjö <martin@martin.st>

configure

@@ -2354,6 +2354,8 @@ msvc_flags(){
             -fno-common)          ;;
             -fno-signed-zeros)    ;;
             -fPIC)                ;;
+            -mthumb)              ;;
+            -march=*)             ;;
             -lz)                  echo zlib.lib ;;
             -lavifil32)           echo vfw32.lib ;;
             -lavicap32)           echo vfw32.lib user32.lib ;;