[testsuite] Pass pthreads in prepare_for_testing

[testsuite] Pass pthreads in prepare_for_testing

"pthreads" in the right flag to pass in prepare_for_testing to linker,
instead of additional_flags.  Without this patch, the test case can't be
complied by clang.

gdb compile failed, clang: warning: -lpthread: 'linker' input unused

gdb/testsuite:

2017-11-22  Yao Qi  <yao.qi@linaro.org>

	* gdb.base/info-os.exp: Pass pthreads.
	* gdb.multi/multi-attach.exp: Likewise.