fix _crc32c used in loading.jl

[KristofferC]

Previously:

julia> crc, x = 0x6635934445471676, 0x5202ff58;

julia> Base._crc32c(crc, x)
0x8b058d60

julia> Base._crc32c(crc, x)
0x2bcaa1e5

julia> Base._crc32c(crc, x)
0xdc2d7c70

Problem is that Vector instead of Ptr was used. Here I just simplified this to handle the return value of hash (UInt64) which is the only thing we use (this also avoids forming the [x]).

Fixes #38101.

[staticfloat]

Oops. Why is this a problem?

[KristofferC]

The C function expects a pointer so giving it a julia array likely makes it read the wrong stuff.

[staticfloat]

Oh, of course, right, the type passed to ccall() needs to be Ptr{eltype} (or Ref{eltype}) in order to skip the Julia object metadata, not Vector{eltype}.