Backport #3074 to 4.1: Don't use Clang nullability qualifiers in strict ISO C mode

[soutaro]

Backports #3074 to the aaa-4.1.x branch, cherry-picked with -x.

_Nullable and _Nonnull are Clang extensions, so compiling the rbs headers in a strict ISO C dialect emits -Wnullability-extension, which -pedantic-errors turns into an error:

include/rbs/ast.h:166:44: error: type nullability specifier '_Nonnull' is a Clang extension [-Werror,-Wnullability-extension]
  166 | void rbs_node_list_append(rbs_node_list_t *RBS_NONNULL list, rbs_node_t *RBS_NONNULL node);
      |                                            ^
include/rbs/defines.h:93:21: note: expanded from macro 'RBS_NONNULL'
   93 | #define RBS_NONNULL _Nonnull

CRuby's omnibus compilations, #5 job builds with -std=c99 -Werror=pedantic -pedantic-errors (and c11/c17/c2x), so ext/rbs_extension fails to compile there once rbs 4.1.x is bundled — see ruby/ruby#18274. rbs 4.0.3 had no nullability macros, so this is new in 4.1.x, which makes the fix only reach CRuby through this branch.

The macros expand to nothing when __STRICT_ANSI__ is defined, which is exactly the case where the qualifiers are rejected. Clang's default -std=gnu* dialects keep the annotations, so nothing is lost for the static analyzer.

The cherry-pick applied cleanly with no conflicts.

Verification

• clang -std=c99 -pedantic-errors -Werror on src/ast.c fails on aaa-4.1.x as it stands and passes with this commit, and the same holds for -std=c11 and -std=c17.
• rake compile and a parser smoke test pass.

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