gendwarfksyms: Add support for reserved and ignored fields

Distributions that want to maintain a stable kABI need the ability
to make ABI compatible changes to kernel data structures without
affecting symbol versions, either because of LTS updates or backports.

With genksyms, developers would typically hide these changes from
version calculation with #ifndef __GENKSYMS__, which would result
in the symbol version not changing even though the actual type has
changed. When we process precompiled object files, this isn't an
option.

Change union processing to recognize field name prefixes that allow
the user to ignore the union completely during symbol versioning with
a __kabi_ignored prefix in a field name, or to replace the type of a
placeholder field using a __kabi_reserved field name prefix.

For example, assume we want to add a new field to an existing
alignment hole in a data structure, and ignore the new field when
calculating symbol versions:

struct struct1 {
int a;
/* a 4-byte alignment hole */
unsigned long b;
};

To add `int n` to the alignment hole, we can add a union that includes
a __kabi_ignored field that causes gendwarfksyms to ignore the entire
union:

struct struct1 {
int a;
union {
char __kabi_ignored_0;
int n;
};
unsigned long b;
};

With --stable, both structs produce the same symbol version.

Alternatively, when a distribution expects future modification to a
data structure, they can explicitly add reserved fields:

struct struct2 {
long a;
long __kabi_reserved_0; /* reserved for future use */
};

To take the field into use, we can again replace it with a union, with
one of the fields keeping the __kabi_reserved name prefix to indicate
the original type:

struct struct2 {
long a;
union {
long __kabi_reserved_0;
struct {
int b;
int v;
};
};

Here gendwarfksyms --stable replaces the union with the type of the
placeholder field when calculating versions.

Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Reviewed-by: Petr Pavlu <petr.pavlu@suse.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>