icall: Use mono_strtod on non-Android/non-iOS ARM platforms

Mono's Double.Parse uses mono_strtod (a non-locale-aware version of
strtod) on most platforms, except on ARM, where it directly calls the
platform's strtod.

This changes the compile-time conditional to (__arm__ && (ANDROID ||
IOS)), to avoid locale-related issues on other ARM operating systems,
notably Tizen, where the strtod implementation is more complete.

This is a more conservative version of a change which has already been
committed to `master` in commit `abb4701`--which completely removes
the conditional.