semihosting: Split out semihost_sys_open

semihosting: Split out semihost_sys_open

Split out the non-ARM specific portions of SYS_OPEN to a
reusable function.  This handles gdb and host file i/o.

Add helpers to validate the length of the filename string.
Prepare for usage by other semihosting by allowing the
filename length parameter to be 0, and calling strlen.

Reviewed-by: Luc Michel <lmichel@kalray.eu>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>