powerpc/kasan: Disable KCOV in KASAN code

As per the generic KASAN code in mm/kasan, disable KCOV with
KCOV_INSTRUMENT := n in the makefile.

This fixes a ppc64 boot hang when KCOV and KASAN are enabled.
kasan_early_init() gets called before a PACA is initialised, but the
KCOV hook expects a valid PACA.

Suggested-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Benjamin Gray <bgray@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230710044143.146840-1-bgray@linux.ibm.com