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