Fix x_int64.c

Clearing a misunderstanding.  The routines c2i_uint64_int() and
i2c_uint64_int() expect to receive that internal values are absolute
and with a separate sign flag, and the x_int64.c code handles values
that aren't absolute and have the sign bit embedded.  We therefore
need to convert between absolute and non-absolute values for the
encoding of negative values to be correct.

[extended tests]

Reviewed-by: Andy Polyakov <appro@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3160)
(cherry picked from commit 429223d1)