codegen: fix bad declaration of memcmp stdlib function signature

[vtjnash]

We were lying to LLVM, and it now catches that in the verifier. Stop
lying to LLVM so that it will again be happy to generate good code for
us (e.g. optimizing this to bcmp).

Aside: the code we emit for the failing test was quite bad. Also, it's worthwhile taking a look at how we handle a slight variation on the original test:

julia> struct LargeStruct
           x::NTuple{1024,Int8}
           LargeStruct() = new()
       end
julia> const large_struct = LargeStruct()
julia> @noinline create_large_struct() = Base.inferencebarrier(large_struct)
julia> function compare_large_struct(a)
           a = a::LargeStruct
           b = create_large_struct()::LargeStruct
           GC.gc() # compare with and without this barrier
           return a === b
       end
julia> code_llvm(compare_large_struct, Tuple{Any}, optimize=false)
julia> code_llvm(compare_large_struct, Tuple{Any}, optimize=true)