It was incorrect to subtract off the size of an unsigned int here.  In
bsd-user fork, this change was made when moving the arch specific items
to specific files.  The size in BSD that's available for the arguments
does not need a return address subtracted from it.

Signed-off-by: Warner Losh <imp@bsdimp.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>