gcov: Remove old GCC 3.4 support

The kernel requires at least GCC 4.8 in order to build, and so there is
no need to cater for the pre-4.7 gcov format.

Remove the obsolete code.

Acked-by: Peter Oberparleiter <oberpar@linux.ibm.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Will Deacon <will@kernel.org>