checkpatch: special case extern struct in .c

"externs should be avoided in .c files" needs an exception for linker
symbols, like those that mark the start, stop of many kernel sections.

Since checkpatch already checks REALNAME to avoid looking at fragments
changing vmlinux.lds.h, add a new else-if block to look at them
instead.  As a simple heuristic, treat all words (in the patch-line)
as possible symbols, to screen later warnings.

For my test case, the possible-symbols included BOUNDED_BY (a macro),
which is extra, but not troublesome - these are just to screen
WARNINGS that might be issued on later fragments (changing .c files)

Where the WARN is issued, precede it with an else-if block to catch
one common extern-in-c use case: "extern struct foo bar[]".  Here we
can at least issue a softer warning, after checking for a match with a
maybe-linker-symbol parsed earlier from the patch.

Though heuristic, it worked for my test-case, allowing both start__,
stop__ $symbol's (wo the prefixes specifically named).  I've coded it
narrowly, it can be expanded later to cover any other expressions.

It does require that the externs in .c's have the additions to
vmlinux.lds.h in the same patch.  And requires vmlinux.lds.h before .c
fragments.

Link: https://lkml.kernel.org/r/20230808033019.21911-2-jim.cromie@gmail.com
Signed-off-by: Jim Cromie <jim.cromie@gmail.com>
Cc: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>