32-bit platform support

[arkpar]

dag_sizes in C implementation overflows with 32-bit size_t type. There might be other issues as well, see cpp-ethereum issue

[jorisbontje]

getting the following build warnings:

# github.com/ethereum/ethash
In file included from Godeps/_workspace/src/github.com/ethereum/ethash/src/libethash/internal.c:30:0,
                 from Godeps/_workspace/src/github.com/ethereum/ethash/ethash.go:16:
Godeps/_workspace/src/github.com/ethereum/ethash/src/libethash/data_sizes.h:129:9: warning: large integer implicitly truncated to unsigned type [-Woverflow]
         4303349632U, 4311743104U, 4320130432U, 4328521088U, 4336909184U,
         ^
Godeps/_workspace/src/github.com/ethereum/ethash/src/libethash/data_sizes.h:129:9: warning: large integer implicitly truncated to unsigned type [-Woverflow]
Godeps/_workspace/src/github.com/ethereum/ethash/src/libethash/data_sizes.h:129:9: warning: large integer implicitly truncated to unsigned type [-Woverflow]
Godeps/_workspace/src/github.com/ethereum/ethash/src/libethash/data_sizes.h:129:9: warning: large integer implicitly truncated to unsigned type [-Woverflow]
Godeps/_workspace/src/github.com/ethereum/ethash/src/libethash/data_sizes.h:129:9: warning: large integer implicitly truncated to unsigned type [-Woverflow]
Godeps/_workspace/src/github.com/ethereum/ethash/src/libethash/data_sizes.h:130:9: warning: large integer implicitly truncated to unsigned type [-Woverflow]
         4345295488U, 4353687424U, 4362073472U, 4370458496U, 4378852736U,