Fix bitshift overflow error on 32-bit archs

When compiling libsodium for a 32-bit architecture (in my case, armv7s, compiled with clang), throws the following warning:

    libsodium/src/libsodium/crypto_scalarmult/curve25519/donna_c64/smult.c:45:41: warning: shift count >= width of type [-Wshift-count-overflow]

This commit fixes this by changes 1l (which is only guaranteed to be 32-bit) into 1ll which is a 64-bit value.