This changes the qemu signal frame layout to be more like the kernel's,
in that the various records are dynamically allocated rather than fixed
in place by a structure.

For now, all of the allocation is out of uc.tuc_mcontext.__reserved,
so the allocation is actually trivial.  That will change with SVE support.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20180303143823.27055-4-richard.henderson@linaro.org
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>