kfence: fix reports if constant function prefixes exist

kfence: fix reports if constant function prefixes exist

Some architectures prefix all functions with a constant string ('.' on
ppc64). Add ARCH_FUNC_PREFIX, which may optionally be defined in
<asm/kfence.h>, so that get_stack_skipnr() can work properly.

Link: https://lkml.kernel.org/r/f036c53d-7e81-763c-47f4-6024c6c5f058@csgroup.eu
Reported-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Tested-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Marco Elver <elver@google.com>
[port kfence feature to rpi-5.10.95]
Signed-off-by: Sung-hun Kim <sfoon.kim@samsung.com>
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Seung-Woo Kim <sw0312.kim@samsung.com>
Change-Id: If0898f06ef741cc25dc0eaed3c4e2095ffe420f5