tools/nolibc: compiler: introduce __nolibc_has_attribute()

Recent compilers support __has_attribute() to check if a certain
compiler attribute is supported.
Unfortunately we have to first check if __has_attribute is supported in
the first place and then if a specific attribute is present.
These two checks can't be folded into a single condition as that would
lead to errors.

Nesting the two conditions like below works, but becomes ugly as soon
as #else blocks are used as those need to be duplicated for both levels
of #if.

#if defined __has_attribute
# if __has_attribute (nonnull)
# define ATTR_NONNULL __attribute__ ((nonnull))
# endif
#endif

Introduce a new helper which makes the usage of __has_attribute() nicer
and migrate the current user to it.

Acked-by: Willy Tarreau <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Thomas Weißschuh <[e-mail]>

tools/include/nolibc/compiler.h

···
 #ifndef _NOLIBC_COMPILER_H
 #define _NOLIBC_COMPILER_H
 
+#if defined(__has_attribute)
+#  define __nolibc_has_attribute(attr) __has_attribute(attr)
+#else
+#  define __nolibc_has_attribute(attr) 0
+#endif
+
 #if defined(__SSP__) || defined(__SSP_STRONG__) || defined(__SSP_ALL__) || defined(__SSP_EXPLICIT__)
 
 #define _NOLIBC_STACKPROTECTOR
 
 #endif /* defined(__SSP__) ... */
 
-#if defined(__has_attribute)
-#  if __has_attribute(no_stack_protector)
-#    define __no_stack_protector __attribute__((no_stack_protector))
-#  else
-#    define __no_stack_protector __attribute__((__optimize__("-fno-stack-protector")))
-#  endif
+#if __nolibc_has_attribute(no_stack_protector)
+#  define __no_stack_protector __attribute__((no_stack_protector))
 #else
 #  define __no_stack_protector __attribute__((__optimize__("-fno-stack-protector")))
-#endif /* defined(__has_attribute) */
+#endif /* __nolibc_has_attribute(no_stack_protector) */
 
 #endif /* _NOLIBC_COMPILER_H */