gcov: move gcov structs definitions to a gcc version specific file

Since also the gcov structures(gcov_info, gcov_fn_info, gcov_ctr_info) can
change between gcc releases, as shown in gcc 4.7, they cannot be defined
in a common header and need to be moved to a specific gcc implemention
file.  This also requires to make the gcov_info structure opaque for the
common code and to introduce simple helpers for accessing data inside
gcov_info.

Signed-off-by: Frantisek Hrbata <fhrbata@redhat.com>
Cc: Jan Stancek <jstancek@redhat.com>
Cc: Kees Cook <keescook@chromium.org>
Acked-by: Peter Oberparleiter <peter.oberparleiter@de.ibm.com>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Andy Gospodarek <agospoda@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>