include/exec/helper-head.h: support "const void *" in helper calls

include/exec/helper-head.h: support "const void *" in helper calls

Especially when dealing with out-of-line gvec helpers, it is often
helpful to specify some vector pointers as constant. E.g. when
we have two inputs and one output, marking the two inputs as consts
pointers helps to avoid bugs.

Const pointers can be specified via "cptr", however behave in TCG just
like ordinary pointers. We can specify helpers like:

DEF_HELPER_FLAGS_4(gvec_vbperm, TCG_CALL_NO_RWG, void, ptr, cptr, cptr, i32)

void HELPER(gvec_vbperm)(void *v1, const void *v2, const void *v3,
                         uint32_t desc)

And make sure that here, only v1 will be written (as long as const is
not casted away, of course).

Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20190221093459.22547-1-david@redhat.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>