If converting a negative double to an UINT64, on arm it's clipped
to zero instead of converted to its two's complement form (contrary
to x86).

This fixes cases like pow(2.0, -2.0) on arm/aarch64. Add similar
casts to expf() too which seems to have a similar pattern, even if
it's not known if that case can cause issues or not.

In the original musl source, the converttoint() function returns a
signed int32_t, doing the same double->signed->unsigned conversion,
implicitly.
Signed-off-by: Martin Storsjo <martin@martin.st>
Signed-off-by: Piotr Caban <piotr@codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>