scripts/mksysmap: Ignore __pi_ local arm64 symbols

Similarly to "__kvm_nvhe_", filter out any local symbol that was
prefixed with "__pi_" (generated when CONFIG_RANDOMIZE_BASE=y) when
compiling System.map and in kallsyms.

Signed-off-by: Pierre-Clément Tosi <ptosi@google.com>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>