target/i386: introduce gen_lea_ss_ofs

Generalize gen_stack_A0() to include an initial add and to use an arbitrary
destination.  This is a common pattern and it is not a huge burden to
add the extra arguments to the only caller of gen_stack_A0().

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>