semihosting: Split out semihost_sys_read

semihosting: Split out semihost_sys_read

Split out the non-ARM specific portions of SYS_READ to a
reusable function.  This handles all GuestFD.  Isolate the
curious ARM-specific return value processing to a new
callback, common_semi_rw_cb.

Note that gdb_do_syscall %x reads target_ulong, not int.

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