powerpc/mm: Disable kcov for SLB routines

The kcov instrumentation inside SLB routines causes duplicate SLB
entries to be added resulting into SLB multihit machine checks.
Disable kcov instrumentation on slb.o

Signed-off-by: Mahesh Salgaonkar <mahesh@linux.vnet.ibm.com>
Acked-by: Andrew Donnellan <andrew.donnellan@au1.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>