[testsuite] Pass -pie in ldflags

[testsuite] Pass -pie in ldflags

-pie is a linker flag, it should be passed via "ldflags", instead
of "additional_flags".  Otherwise, clang complains,

clang: warning: argument unused during compilation: '-pie'

gdb/testsuite:

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

	* gdb.base/attach-pie-noexec.exp: Pass "-pie" in ldflags.
	* gdb.base/break-interp.exp: Likewise.
	* gdb.base/jit-attach-pie.exp: Likewise.