linux-user: Honor TARGET_ARCH_HAS_SA_RESTORER in do_syscall

Do not access a field that may not be present.  This will
become an issue when sharing more code in the next patch.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20210422230227.314751-5-richard.henderson@linaro.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>