linux-user: Split out do_syscall1

linux-user: Split out do_syscall1

There was supposed to be a single point of return for do_syscall
so that tracing works properly.  However, there are a few bugs
in that area.  It is significantly simpler to simply split out
an inner function to enforce this.

Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20180818190118.12911-3-richard.henderson@linaro.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>