gcov: Disable gcov build with GCC 10

GCOV built with GCC 10 doesn't initialize n_function variable.  This
produces different kernel panics as was seen by Colin in Ubuntu and me
in FC 32.

As a workaround, let's disable GCOV build for broken GCC 10 version.

Link: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1891288
Link: https://lore.kernel.org/lkml/20200827133932.3338519-1-leon@kernel.org
Link: https://lore.kernel.org/lkml/CAHk-=whbijeSdSvx-Xcr0DPMj0BiwhJ+uiNnDSVZcr_h_kg7UA@mail.gmail.com/


Cc: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>