linux-user: Fix broken m68k signal handling on 64 bit hosts

The m68k signal frame setup code which writes the signal return
trampoline code to the stack was assuming that a 'long' was 32 bits;
on 64 bit systems this meant we would end up writing the 32 bit
(2 insn) trampoline sequence to retaddr+4,retaddr+6 instead of
the intended retaddr+0,retaddr+2, resulting in a guest crash when
it tried to execute the invalid zero-bytes at retaddr+0.
Fix by using uint32_t instead; also use uint16_t rather than short
for consistency. This fixes bug LP:1404690.

Reported-by: Michel Boaventura
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>
