checkasm: x86: properly save rdx/edx in checked_call()

If the return value doesn't fit in a single register rdx/edx can in some
cases be used in addition to rax/eax.

Doesn't affect any of the existing checkasm tests but might be useful later.

Also comment the relevant code a bit better.

Signed-off-by: Anton Khirnov <anton@khirnov.net>