linux-user: Bump do_syscall() up to 8 syscall arguments

linux-user: Bump do_syscall() up to 8 syscall arguments

On 32 bit MIPS a few syscalls have 7 arguments, and so to call
them via NR_syscall the guest needs to be able to pass 8 arguments
to do_syscall(). Raise the number of arguments do_syscall() takes
accordingly.

This fixes some gcc 4.6 compiler warnings about arg7 and arg8
variables being set and never used.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Riku Voipio <riku.voipio@iki.fi>