fix: added arc4random_buf stub

this fixes linking error on aarch64
also _seralPutchar was made public to prevent mangling

Kernel/Source/Support/stubs.c

@@ -34,3 +34,9 @@ int posix_memalign(void** ptr, unsigned long align, unsigned long size) {
 }
 
 void free(void* ptr) { (void)ptr; }
+
+// Swift stdlib uses arc4random_buf for Hasher seed — stub with zeroes in bare-metal
+void arc4random_buf(void* buf, unsigned long nbytes) {
+    unsigned char* b = (unsigned char*)buf;
+    while (nbytes--) *b++ = 0;
+}