Fix: Use malloc and free for cv_stack to avoid dangling pointer warning

[gemenerik]

Before the change, running make cf2_defconfig and make clean generated warnings about dangling pointers in the configuration process.

In function ‘dep_stack_insert’,
    inlined from ‘sym_check_print_recursive’ at ../scripts/kconfig/symbol.c:1103:3,
    inlined from ‘sym_check_deps’ at ../scripts/kconfig/symbol.c:1280:3:
../scripts/kconfig/symbol.c:1079:19: warning: storing the address of local variable ‘cv_stack’ in ‘check_top’ [-Wdangling-pointer=]
 1079 |         check_top = stack;
      |         ~~~~~~~~~~^~~~~~~
../scripts/kconfig/symbol.c: In function ‘sym_check_deps’:
../scripts/kconfig/symbol.c:1100:26: note: ‘cv_stack’ declared here
 1100 |         struct dep_stack cv_stack;
      |                          ^~~~~~~~
../scripts/kconfig/symbol.c:1070:4: note: ‘check_top’ declared here
 1070 | } *check_top;
      |    ^~~~~~~~~

After the update, the command completes without any warnings.

The changes in this PR shouldn't practically change anything except acknowledge that we are responsible for cv_stack. The function in question is called during recursive dependency checking. I tested this PR by deliberately introducing recursive dependency in kconfig.

[ataffanel]

Merged! I cannot verify the full implication of this malloc and if any early free is possible but since this is a diagnostic part of the build system it is unlikely to cause any problem and if it does it will be visible.