[fork] Log when pthread_create fails (#32525)

There was evidence that a `pthread_create` failed in [this
comment](https://github.com/grpc/grpc/issues/31885#issuecomment-1433917201),
but without logging, we couldn't diagnose any further. This PR logs in
this case.

src/core/lib/gprpp/posix/thd.cc

@@ -20,6 +20,8 @@
 
 #include <grpc/support/port_platform.h>
 
+#include <string>
+
 #include <grpc/support/time.h>
 
 #ifdef GPR_POSIX_SYNC
@@ -35,6 +37,7 @@
 
 #include "src/core/lib/gpr/useful.h"
 #include "src/core/lib/gprpp/fork.h"
+#include "src/core/lib/gprpp/strerror.h"
 #include "src/core/lib/gprpp/thd.h"
 
 namespace grpc_core {
@@ -106,7 +109,7 @@ class ThreadInternalsPosix : public internal::ThreadInternalsInterface {
       GPR_ASSERT(pthread_attr_setstacksize(&attr, stack_size) == 0);
     }
 
-    *success = (pthread_create(
+    int pthread_create_err = pthread_create(
         &pthread_id_, &attr,
         [](void* v) -> void* {
           thd_arg arg = *static_cast<thd_arg*>(v);
@@ -144,11 +147,14 @@ class ThreadInternalsPosix : public internal::ThreadInternalsInterface {
           }
           return nullptr;
         },
-                    info) == 0);
+        info);
+    *success = (pthread_create_err == 0);
 
     GPR_ASSERT(pthread_attr_destroy(&attr) == 0);
 
     if (!(*success)) {
+      gpr_log(GPR_ERROR, "pthread_create failed: %s",
+              StrError(pthread_create_err).c_str());
       // don't use gpr_free, as this was allocated using malloc (see above)
       free(info);
       if (options.tracked()) {