Fix race condition in dwarf2_section_info::read

It can occur in dwarf2_section_info::read that:
- thread A sets readin to true, and buffer to nullptr, and then
- thread B reads readin as true, returns and finds that buffer is nullptr
  and concludes that there no such such section.  Which is not true, it's
  just that thread A hasn't finished reading it yet, which would make
  buffer != nullptr.

---
 gdb/dwarf2/section.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/gdb/dwarf2/section.c b/gdb/dwarf2/section.c
index 8e1c0197c85..e6940103d77 100644
--- a/gdb/dwarf2/section.c
+++ b/gdb/dwarf2/section.c
@@ -30,6 +30,8 @@
 #include "objfiles.h"
 #include "complaints.h"
 
+#include <mutex>
+
 void
 dwarf2_section_info::overflow_complaint () const
 {
@@ -118,10 +120,12 @@ dwarf2_section_info::empty () const
 void
 dwarf2_section_info::read (struct objfile *objfile)
 {
+  static std::mutex mutex;
   asection *sectp;
   bfd *abfd;
   gdb_byte *buf, *retbuf;
 
+  std::lock_guard<std::mutex> guard (mutex);
   if (readin)
     return;
   buffer = NULL;