net: rds: add option for GCOV profiling

To better our unit tests we need code coverage to be part of the kernel.
This patch borrows heavily from how CONFIG_GCOV_PROFILE_FTRACE is
implemented

Reviewed-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
Signed-off-by: Allison Henderson <allison.henderson@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>