timeval: Avoid backtrace() from signal handler on x86-64.

timeval: Avoid backtrace() from signal handler on x86-64.

backtrace() is really useful, but it is not signal safe everywhere.  We
need to reassess whether it is reasonable to use it anywhere, but
immediately we need to disable it on x86-64 (with glibc) because it is
causing segfaults in testing.

Bug #15497.
Reported-by: Ram Jothikumar <rjothikumar@vmware.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Ethan Jackson <ethan@nicira.com>