Update shared libraries version info to differentiate against v7.4.x

Update shared libraries version info to differentiate against v7.4.x

Version info "current" and "age" numbers are incremented, "revision"
number is reset to zero (indicating that programs using the previous
libraries version may use the new version as drop-in replacement).
Thus, version info is 2:0:1 now (both for libatomic_ops.so and
libatomic_ops_gpl.so).

* src/Makefile.am (LIBATOMIC_OPS_VER_INFO, LIBATOMIC_OPS_GPL_VER_INFO):
Change version info from 1:3:0 (used for libatomic_ops v7.4.x releases)
to 2:0:1.