linux-user/sparc: Fix errors in target_ucontext structures

linux-user/sparc: Fix errors in target_ucontext structures

The various structs that make up the SPARC target_ucontext had some
errors:
 * target structures must not include fields which are host pointers,
   which might be the wrong size.  These should be abi_ulong instead
 * because we don't have the 'long double' part of the mcfpu_fregs
   union in our version of the target_mc_fpu struct, we need to
   manually force it to be 16-aligned

In particular, the lack of 16-alignment caused sparc64_get_context()
and sparc64_set_context() to read and write all the registers at the
wrong offset, which triggered a guest glibc stack check in
siglongjmp:
  *** longjmp causes uninitialized stack frame ***: terminated
when trying to run bash.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20201105212314.9628-2-peter.maydell@linaro.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>