gcov: gcc_4_7: replace zero-length array with flexible-array member

The current codebase makes use of the zero-length array language extension
to the C90 standard, but the preferred mechanism to declare
variable-length types such as these ones is a flexible array member[1][2],
introduced in C99:

struct foo {
        int stuff;
        struct boo array[];
};

By making use of the mechanism above, we will get a compiler warning in
case the flexible array does not occur last in the structure, which will
help us prevent some kind of undefined behavior bugs from being
inadvertently introduced[3] to the codebase from now on.

Also, notice that, dynamic memory allocations won't be affected by this
change:

"Flexible array members have incomplete type, and so the sizeof operator
may not be applied.  As a quirk of the original implementation of
zero-length arrays, sizeof evaluates to zero."[1]

This issue was found with the help of Coccinelle.

[1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
[2] KSPP#21
[3] commit 7649773 ("cxgb3/l2t: Fix undefined behaviour")

Signed-off-by: Gustavo A. R. Silva <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Acked-by: Peter Oberparleiter <[email protected]>
Link: http:https://lkml.kernel.org/r/20200213152241.GA877@embeddedor
Signed-off-by: Linus Torvalds <[email protected]>

kernel/gcov/gcc_4_7.c

@@ -68,7 +68,7 @@ struct gcov_fn_info {
  unsigned int ident;
  unsigned int lineno_checksum;
  unsigned int cfg_checksum;
- struct gcov_ctr_info ctrs[0];
+ struct gcov_ctr_info ctrs[];
 };
 
 /**