kbuild: fix CONFIG_TRIM_UNUSED_KSYMS build for ppc64

Stephen Rothwell reported a build error on ppc64 when
CONFIG_TRIM_UNUSED_KSYMS is enabled.

Jessica Yu pointed out the cause of the error with the reference to the
ppc64 ELF ABI:
"Symbol names with a dot (.) prefix are reserved for holding entry
point addresses. The value of a symbol named ".FN", if it exists,
is the entry point of the function "FN".

As it turned out, CONFIG_TRIM_UNUSED_KSYMS has never worked for ppc64,
but this issue has been unnoticed until recently because this option
depends on !UNUSED_SYMBOLS hence is disabled by all{mod,yes}config.
(Then, it was uncovered by another patch removing UNUSED_SYMBOLS.)

Removing the dot prefix in scripts/gen_autoksyms.sh fixes the issue.
Please note it must be done before 'sort -u' because modules have
both ._mcount and _mcount undefined when CONFIG_FUNCTION_TRACER=y.

Link: https://lore.kernel.org/lkml/[e-mail]/
Reported-by: Stephen Rothwell <[e-mail]>
Signed-off-by: Masahiro Yamada <[e-mail]>
Tested-by: Jessica Yu <[e-mail]>

scripts/gen_autoksyms.sh

···
 sed 's/ko$/mod/' $modlist |
 xargs -n1 sed -n -e '2{s/ /\n/g;/^$/!p;}' -- |
 cat - "$ksym_wl" |
+# Remove the dot prefix for ppc64; symbol names with a dot (.) hold entry
+# point addresses.
+sed -e 's/^\.//' |
 sort -u |
 sed -e 's/\(.*\)/#define __KSYM_\1 1/' >> "$output_file"