bash: fix mkbuiltins build failure

bash: fix mkbuiltins build failure

On hosts with FORTIFY_SOURCES, stringize support is required, as it's used by
the macros to wrap functions (e.g. read and open in unistd.h). Those wrappers
use the STRING() macro from unistd.h. A header in the bash sources overrides
the unistd.h macro to 'x' when HAVE_STRINGIZE is not defined, causing the
wrappers to generate calls to 'xread' and 'xopen', which do not exist,
resulting in a failure to link.

Assume we have stringize support when cross-compiling, which works around the
issue.

It may be best for upstream to either give up on supporting compilers without
stringize support, or to not define STRING() at all when FORTIFY_SOURCES is
defined, letting the unistd.h one be used, instead.

(From OE-Core rev: f7a25dd72d1d463eb72d48c6f9dd968d376496c0)

Signed-off-by: Christopher Larson <chris_larson@mentor.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>