Block signals in pthread_create.

HWASan-instrumented code needs TLS_SLOT_SANITIZER set up to run, and
that is not done until the new thread calls __hwasan_thread_enter. Block
all signals until that time to prevent hwasan-instrumented signal
handlers running (and crashing) on the new thread.

Bug: 141893397
Test: seq 0 10000000 | xargs -n 1 -P 200 adb shell am instrument \
      -w -r -e command grant-all \
      com.android.permissionutils/.PermissionInstrumentation

Change-Id: I63ec7080cd5b0ee8dcd6ae2ec5a90a19c83cfe4e
diff --git a/libc/bionic/pthread_internal.h b/libc/bionic/pthread_internal.h
index 22cc400..22b0558 100644
--- a/libc/bionic/pthread_internal.h
+++ b/libc/bionic/pthread_internal.h
@@ -98,6 +98,7 @@
   void* (*start_routine)(void*);
   void* start_routine_arg;
   void* return_value;
+  sigset64_t start_mask;
 
   void* alternate_signal_stack;