[gdb/testsuite] Fail in gdb_compile if nopie results in PIE executable

[gdb/testsuite] Fail in gdb_compile if nopie results in PIE executable

When running gdb.base/dump.exp with --target_board=unix/-fPIE/-pie, we get:
...
Running gdb/testsuite/gdb.base/dump.exp ...
FAIL: gdb.base/dump.exp: dump array as value, intel hex
...

The FAIL happens because although the test specifies nopie, the exec is
in fact compiled as PIE.  The "-fPIE -pie" options specified using the
target_board are interpreted by dejagnu as multilib_flags, and end up
overriding the nopie flags.

Fix this by checking in gdb_compile if the resulting exec is PIE despite of
a nopie setting, and if so return an error:
...
Running gdb/testsuite/gdb.base/dump.exp ...
gdb compile failed, nopie failed to prevent PIE executable

                === gdb Summary ===

nr of untested testcases         1
...

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2019-07-30  Tom de Vries  <tdevries@suse.de>

	PR testsuite/24834
	* lib/gdb.exp (gdb_compile): Fail if nopie results in PIE executable.
	(exec_is_pie): New proc.