meson: fix libatomic tests

There are two problems:
1) the extra underscore in MISSING_64BIT_ATOMICS
2) we should link with libatomic if the previous test decided we needed
   it

Fixes: d1992255bb
       ("meson: Add build Intel "anv" vulkan driver")
Reviewed-and-Tested-by: Matt Turner <mattst88@gmail.com>

meson.build

@@ -905,8 +905,9 @@ if not cc.links('''#include <stdint.h>
                    int main() {
                      return __sync_add_and_fetch(&v, (uint64_t)1);
                    }''',
+                dependencies : dep_atomic,
                 name : 'GCC 64bit atomics')
-  pre_args += '-DMISSING_64_BIT_ATOMICS'
+  pre_args += '-DMISSING_64BIT_ATOMICS'
 endif
 
 # TODO: shared/static? Is this even worth doing?