arm-semi: Provide access to CLI arguments passed through the "-append" option

This patch basically adapts the new semi-hosting command-line support
-- introduced by Wolfgang Schildbach in the commit 2e8785ac -- for use
in system-mode.

Note that the "arm_cmdline_len" and "host_cmdline_len" variables were
renamed respectively "input_size" and "output_size" because:

    * in C, the term "length" is generally used to count the number of
      character in a string, not to count the number of bytes in a
      buffer (as it is the case here).

    * in QEMU, the term "host" is used to name variables that are in
      the host address space, not to name variables in the target
      address space (as it is the case here).

    * in the case of this system-call, the terms "input" and "output"
      fit the semantic of the official ARM semi-hosting specification
      quite well.

I know renaming can be considered harmful but I do think in this case
the semantic really matters to keep this code more understandable.

Signed-off-by: Cédric VINCENT <cedric.vincent@st.com>
Reviewed-by: Christophe Lyon <christophe.lyon@st.com>
Cc: Peter Maydell <peter.maydell@linaro.org>
Cc: Paul Brook <paul@codesourcery.com>
Cc: Wolfgang Schildbach <wschi@dolby.com>
Cc: Riku Voipio <riku.voipio@iki.fi>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>