kbuild,gcov: simplify kernel/gcov/Makefile more

CONFIG_GCOV_FORMAT_3_4 / _4_7 / _AUTODETECT are exclusive.
Compare the CC version only when _AUTODETECT is enabled.

This change should have no impact.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Peter Oberparleiter <oberpar@linux.vnet.ibm.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>