Compiler Attributes: add support for __nonstring (gcc >= 8)

Compiler Attributes: add support for __nonstring (gcc >= 8)

From the GCC manual:

  nonstring

    The nonstring variable attribute specifies that an object or member
    declaration with type array of char, signed char, or unsigned char,
    or pointer to such a type is intended to store character arrays that
    do not necessarily contain a terminating NUL. This is useful in detecting
    uses of such arrays or pointers with functions that expect NUL-terminated
    strings, and to avoid warnings when such an array or pointer is used as
    an argument to a bounded string manipulation function such as strncpy.

  https://gcc.gnu.org/onlinedocs/gcc/Common-Variable-Attributes.html

This attribute can be used for documentation purposes (i.e. replacing
comments), but it is most helpful when the following warnings are enabled:

  -Wstringop-overflow

    Warn for calls to string manipulation functions such as memcpy and
    strcpy that are determined to overflow the destination buffer.

    [...]

  -Wstringop-truncation

    Warn for calls to bounded string manipulation functions such as
    strncat, strncpy, and stpncpy that may either truncate the copied
    string or leave the destination unchanged.

    [...]

    In situations where a character array is intended to store a sequence
    of bytes with no terminating NUL such an array may be annotated with
    attribute nonstring to avoid this warning. Such arrays, however,
    are not suitable arguments to functions that expect NUL-terminated
    strings. To help detect accidental misuses of such arrays GCC issues
    warnings unless it can prove that the use is safe.

  https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html

Tested-by: Sedat Dilek <sedat.dilek@gmail.com> # on top of v4.19-rc5, clang 7
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>
Signed-off-by: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>