linux-user: Add signal handling support for x86_64

linux-user: Add signal handling support for x86_64

Note that x86_64 has only _rt signal handlers. This implementation
attempts to share code with the x86_32 implementation.

CC: Laurent Vivier <laurent@vivier.eu>
Signed-off-by: Allan Wirth <awirth@akamai.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20170226165345.8757-1-bobby.prani@gmail.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>