compiler.h: Move C string helpers into C-only kernel section

The C kernel helpers for evaluating C Strings were positioned where they
were visible to assembly inclusion, which was not intended. Move them
into the kernel and C-only area of the header so future changes won't
confuse the assembler.

Fixes: d7a516c6 ("compiler.h: Fix undefined BUILD_BUG_ON_ZERO()")
Fixes: 559048d1 ("string: Check for "nonstring" attribute on strscpy() arguments")
Reviewed-by: Miguel Ojeda <ojeda@kernel.org>
Signed-off-by: Kees Cook <kees@kernel.org>