Some RT signals can be in use by glibc,
it's why SIGRTMIN (34) is generally greater than __SIGRTMIN (32).

So SIGRTMIN cannot be mapped to TARGET_SIGRTMIN.

Instead of swapping only SIGRTMIN and SIGRTMAX, map all the
range [TARGET_SIGRTMIN ... TARGET_SIGRTMAX - X] to
      [__SIGRTMIN + X ... SIGRTMAX ]
(SIGRTMIN is __SIGRTMIN + X).

Signed-off-by: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: Taylor Simson <tsimpson@quicinc.com>
Tested-by: Taylor Simpson <tsimpson@quicinc.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20200212125658.644558-5-laurent@vivier.eu>