MINOR: debug: use our own backtrace function on clang+x86_64

MINOR: debug: use our own backtrace function on clang+x86_64

A test on FreeBSD with clang 4 to 8 produces this on a call to a
spinning loop on the CLI:

  call trace(5):
  |       0x53e2bc [eb 16 48 63 c3 48 c1 e0]: wdt_handler+0x10c
  |    0x800e02cfe [e8 5d 83 00 00 8b 18 8b]: libthr:pthread_sigmask+0x53e

with our own function it correctly produces this:

  call trace(20):
  |       0x53e2dc [eb 16 48 63 c3 48 c1 e0]: wdt_handler+0x10c
  |    0x800e02cfe [e8 5d 83 00 00 8b 18 8b]: libthr:pthread_sigmask+0x53e
  |    0x800e022bf [48 83 c4 38 5b 41 5c 41]: libthr:pthread_getspecific+0xdef
  | 0x7ffffffff003 [48 8d 7c 24 10 6a 00 48]: main+0x7fffffb416f3
  |    0x801373809 [85 c0 0f 84 6f ff ff ff]: libc:__sys_gettimeofday+0x199
  |    0x801373709 [89 c3 85 c0 75 a6 48 8b]: libc:__sys_gettimeofday+0x99
  |    0x801371c62 [83 f8 4e 75 0f 48 89 df]: libc:gettimeofday+0x12
  |       0x51fa0a [48 89 df 4c 89 f6 e8 6b]: ha_thread_dump_all_to_trash+0x49a
  |       0x4b723b [85 c0 75 09 49 8b 04 24]: mworker_cli_sockpair_new+0xd9b
  |       0x4b6c68 [85 c0 75 08 4c 89 ef e8]: mworker_cli_sockpair_new+0x7c8
  |       0x532f81 [4c 89 e7 48 83 ef 80 41]: task_run_applet+0xe1

So let's add clang+x86_64 to the list of platforms that will use our
simplified version. As a bonus it will not require to link with
-lexecinfo on FreeBSD and will work out of the box when passing
USE_BACKTRACE=1.

(cherry picked from commit 899e5f69a1085983f52a46cd842921874fd2de36)
Signed-off-by: Willy Tarreau <w@1wt.eu>