fortify: Make sure strlen() may still be used as a constant expression

In preparation for enabling Clang FORTIFY_SOURCE support, redefine
strlen() as a macro that tests for being a constant expression
so that strlen() can still be used in static initializers, which is
lost when adding __pass_object_size and __overloadable.

An example of this usage can be seen here:
	https://lore.kernel.org/all/[email protected]/

Notably, this constant expression feature of strlen() is not available
for architectures that build with -ffreestanding. This means the kernel
currently does not universally expect strlen() to be used this way, but
since there _are_ some build configurations that depend on it, retain
the characteristic for Clang FORTIFY_SOURCE builds too.

Signed-off-by: Kees Cook <[email protected]>
Reviewed-by: Nick Desaulniers <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

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