Use BN_set_u64 in NativeBN.

This removes one use of BIGNUM's internals from libcore. (Still have a
ways to go to fix the others, but this one was easy.)

Change-Id: I7d54c18e5301bb519ae6b5f12b87ca7d02a21bd5
Test: cts-tradefed run cts -m CtsLibcoreTestCases
1 file changed