Default dwarf_synchronous to true

Unfortunately the background DWARF reading series introduced a number
of races, as repored by thread sanitizer.  This patch changes gdb to
disable this feature for the time being -- in particular for the gdb
15 release.

I've filed a bug and linked all the known races to it.  Once those are
fixed we can re-enable this feature by default.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31751

gdb/dwarf2/read.c

diff --git a/gdb/dwarf2/read.c b/gdb/dwarf2/read.c
index e841690..4818da5 100644
--- a/gdb/dwarf2/read.c
+++ b/gdb/dwarf2/read.c
@@ -739,7 +739,7 @@ show_dwarf_max_cache_age (struct ui_file *file, int from_tty,
 }
 
 /* When true, wait for DWARF reading to be complete.  */
-static bool dwarf_synchronous = false;
+static bool dwarf_synchronous = true;
 
 /* "Show" callback for "maint set dwarf synchronous".  */
 static void