Stop falling back to 16-bit BignumInt on VS Arm builds.

The case previously conditioned on _M_IX86, where we use __int64 as
the BignumDblInt type, is actually valid on any Visual Studio target
platform at all, so it's safe to remove that condition and let it
apply to _M_ARM and _M_ARM64 as well. The only situation in which we
_shouldn't_ use that case for Visual Studio builds is when we have
something even better available, such as the x86-64 intrinsics for
add-with-carry and double-width multiply.

mpint_i.h

@@ -131,7 +131,7 @@
   #define BIGNUM_INT_BITS_BITS 5
   #define DEFINE_BIGNUMDBLINT typedef unsigned long long BignumDblInt
 
-#elif defined _MSC_VER && defined _M_IX86 && BB_OK(5)
+#elif defined _MSC_VER && BB_OK(5)
 
   /* 32-bit BignumInt, using Visual Studio __int64 as BignumDblInt */