include: ssp: ignore fortify for purecap

lib/libc/secure and hence *_chk() libc function variants for SSP are not
built when the world is compiled for the pure-capability ABI.

In such a case, don't define __SSP_FORTIFY_LEVEL, based on
_FORTIFY_SOURCE, that is used to redefine libc functions to their
*_chk() variants. This check matches the check in lib/libc/Makefile to
exclude lib/libc/secure.

include/ssp/ssp.h

@@ -36,7 +36,7 @@
 
 #include <sys/cdefs.h>
 
-#if !defined(__cplusplus)
+#if !defined(__cplusplus) && !defined(__CHERI_PURE_CAPABILITY__)
 # if defined(_FORTIFY_SOURCE) && _FORTIFY_SOURCE > 0 && \
      (__OPTIMIZE__ > 0 || defined(__clang__))
 #  if _FORTIFY_SOURCE > 1