Correct signature of memmove

[Keno]

wasm was complaining about the incorrect Cvoid rather than Ptr{Void}
in the :memmove calls in ryu, but while we're here also change the
third argument to Csize_t, since that's what the standard says it is.

[vtjnash]

Wasn't the decision in #31464 to commit to detecting this pair of functions as a special case in the backend and make this signature mismatch valid?

[Keno]

Wasn't the decision in #31464 to commit to detecting this pair of functions as a special case in the backend and make this signature mismatch valid?

We certainly decided that for memcpy which we explicitly intercept in ccall.cpp. I'm fine with extending that to memmove also, though of course that would mean adding an appropriate special case in ccall.cpp.