string.h: move fortified functions definitions in a dedicated header.

This patch adds fortify-string.h to contain fortified functions
definitions.  Thus, the code is more separated and compile time is
approximately 1% faster for people who do not set CONFIG_FORTIFY_SOURCE.

Link: https://lkml.kernel.org/r/20210111092141.22946-1-laniel_francis@privacyrequired.com
Link: https://lkml.kernel.org/r/20210111092141.22946-2-laniel_francis@privacyrequired.com


Signed-off-by: Francis Laniel <laniel_francis@privacyrequired.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>