[gdb/testsuite] Fix ada tests with -fPIE/-pie

[gdb/testsuite] Fix ada tests with -fPIE/-pie

When running the gdb testsuite with target board unix/-fPIE/-pie, the
resulting ada executables are not PIE executables, because gnatmake doesn't
recognize -pie, and consequently doesn't pass it to gnatlink.

Fix this by replacing "-pie" with "-largs -pie -margs" in
target_compile_ada_from_dir, and doing the same for -no-pie.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2019-10-10  Tom de Vries  <tdevries@suse.de>

	PR testsuite/24888
	* lib/ada.exp (target_compile_ada_from_dir): Route -pie/-no-pie to
	gnatlink.