tree-optimization/105043: Object Size Checking docs cleanup
Break the _FORTIFY_SOURCE-specific builtins out into a separate
subsection from Object Size Checking built-ins and mention
_FORTIFY_SOURCE in there so that the link between the object size
checking builtins, the helper builtins (e.g. __builtin___memcpy_chk) and
_FORTIFY_SOURCE is clearer.
gcc/ChangeLog:
PR tree-optimization/105043
* doc/extend.texi (Object Size Checking): Split out into two
subsections and mention _FORTIFY_SOURCE.
Signed-off-by: Siddhesh Poyarekar <siddhesh@gotplt.org>