Harden gdb.arch/aarch64-pauth.exp and fix a failure

When running this testcase against a QEMU with PAC support, i noticed we
were failing to recognize the additional [PAC] that is emitted in the
backtrace, resulting in this failure:

FAIL: gdb.arch/aarch64-pauth.exp: backtrace

I've made the test use multi_line to make the pattern more clear.

Tested against aarch64-linux-gnu with and without PAC support.

gdb/testsuite/ChangeLog:

2020-02-28  Luis Machado  <luis.machado@linaro.org>

	* gdb.arch/aarch64-pauth.exp: Recognize optional PAC output.