[vm] Take only a read lock during Field::SetStaticValue.

It is safe for multiple isolates to set their values concurrently. Only growth of the field tables needs to be excluded by the write lock.

A storm of such setters are invoked as part of core library initialization during isolate spawning.

TEST=ci, tsan
Change-Id: I4677e33ee80bb06d261ada2ae9d6b71c94fde611
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/405400
Commit-Queue: Ryan Macnak <rmacnak@google.com>
Reviewed-by: Alexander Aprelev <aam@google.com>

runtime/vm/object.cc

diff --git a/runtime/vm/object.cc b/runtime/vm/object.cc
index b909685..a02f753 100644
--- a/runtime/vm/object.cc
+++ b/runtime/vm/object.cc
@@ -13330,7 +13330,7 @@
   const intptr_t id = field_id();
   ASSERT(id >= 0);
 
-  SafepointWriteRwLocker ml(thread, thread->isolate_group()->program_lock());
+  SafepointReadRwLocker ml(thread, thread->isolate_group()->program_lock());
   thread->isolate()->field_table()->SetAt(id, value.ptr());
 }