testsuite: Fix gdb.btrace/tailcall-only.exp errors on x86_64-m32

testsuite: Fix gdb.btrace/tailcall-only.exp errors on x86_64-m32

$ runtest 'CC_FOR_TARGET=gcc -m32' gdb.btrace/tailcall-only.exp
Running ./gdb.btrace/tailcall-only.exp ...
gdb compile failed, tailcall-only.c: Assembler messages:
tailcall-only.c:142: Error: cannot represent relocation type BFD_RELOC_64
[...]
tailcall-only.c:425: Error: cannot represent relocation type BFD_RELOC_64

It works for the other x86 arch combinations:

On Mon, 11 Apr 2016 08:44:23 +0200, Metzger, Markus T wrote:
I'm setting the target triplet to "i686-unknown-linux" in my m32 configuration.
Like this:

set target_triplet "i686-unknown-linux"
set_board_info cflags "-m32"
set_board_info cppflags "-m32"

On Wed, 20 Jul 2016 16:02:20 +0200, Pedro Alves wrote:
There's no reason you should _not_ set it.

But, multilib-style testing with --target_board=unix\{-m64,-m32\} etc.
should work _too_, IMO.

gdb/testsuite/ChangeLog
2016-07-20  Jan Kratochvil  <jan.kratochvil@redhat.com>

	* gdb.btrace/tailcall-only.exp: Use is_lp64_target check.