aix: guard STATIC_ASSERT for glibc work around

On 64-bit AIX `sizeof(uv_sem_t)` is 4 bytes which is not large
enough to store a pointer. AIX doesn't use glibc so the work around
introduced by #1795 doesn't
apply, so guard the STATIC_ASSERT so that it is only used when the
custom semaphore implementation is used.

Refs: nodejs/node#20129
Refs: #1795
PR-URL: #1808
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

src/unix/thread.c

@@ -449,7 +449,7 @@ static void glibc_version_check(void) {
 
 #define platform_needs_custom_semaphore 1
 
-#else /* !defined(__GLIBC__) */
+#else /* !defined(__GLIBC__) && !defined(__MVS__) */
 
 #define platform_needs_custom_semaphore 0
 
@@ -461,8 +461,9 @@ typedef struct uv_semaphore_s {
   unsigned int value;
 } uv_semaphore_t;
 
-
+#if defined(__GLIBC__) || platform_needs_custom_semaphore
 STATIC_ASSERT(sizeof(uv_sem_t) >= sizeof(uv_semaphore_t*));
+#endif
 
 static int uv__custom_sem_init(uv_sem_t* sem_, unsigned int value) {
   int err;