MurmurHash3 has undefined behavior

[Keno]

Our implementation of MurmurHash3 has:

julia/src/support/MurmurHash3.c

Lines 60 to 63 in ed37984

	FORCE_INLINE uint64_t getblock64 ( const uint64_t * p, int i )
	{
	return p[i];
	}

However, the pointer is not necessarily properly aligned, making this undefined behavior.
The proper fix is to stop casting to an invalid pointer type and to use memcpy, which all modern compiler optimize to a proper sequence for unaligned access.

This is the same issue as grpc/grpc#13221

[yuyichao]

These can be moved to support.