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 <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Thomas Weißschuh <[email protected]>

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