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>